gdenby

Hi, TBUA,
For doing traditional bit map painting, Photo is a little better than Designer. Both benefit greatly when used w. a tablet.
I infer from your 1st post you mostly work on paper. In Designer, the "pencil" is a specially designed tool to turn hand motion into a geometric vector representation. At this point, Designer does not have a vector eraser. The best on can do is delete individual nodes of the stroke, or break it into smaller pieces. There is an advantage to this. The strokes can be infinitely adjusted, repositioned, duplicated, made to have different thicknesses, etc. Designer has a very nice tool called the vector brush, which works much the same as the pencil, but stretches or repeats a bit-map along the stroke.
Here is an analogy. Working w. vectors is a bit like making a collage, or setting up a mosaic. Erasing really isn't an option. 

Just select 1 curve, and make sure its stroke is set to .5. Use the Edit/copy command, and then select all the others. Then use the edit/paste style command. This will transfer the .5 stroke attribute to all the selected at once. 

As already been pointed out by @firstdefence @gdenby
You gonna have to use the Pixel Persona or Affinity Photo for drawing out what you want to create.
After that using Designer in vector mode to recreate what you drew as vector.
One way I tend to do sometimes is using the geometric tools, and turn them into curves,
and then I use the node tool to push and pull things to where I want them to go.
Example of this

Looking at the drawing, I have to think the different drawing portions are not objects, but groups of curves, or just curves associated by placement in space.. Each curve would have whatever the line weight that the tool that made it had when in use. The line weight panel you show only applies to the stroke as it will be applied currently, not how it was applied before.
I hope that made sense, 'cause I'm inferring a lot from looking at the screen shot.
If you select 1 layer(curve) that does in fact have a .5 stroke, and copy it, the rest of the the curves can be selected, and have "paste style" applied, which will transfer the .5 line stroke attribute to all. 
Its been years since I last used Illustrator, but layers in AD vs. AI do not quite refer to the same thing. If my recollection is correct, an AI layer can be a group of objects placed on a sheet of tracing paper, as it were, to be hidden or exposed as needed. AD layers can also be hidden or exposed groups of object. But there is a hierarchy of effects and clipping within AD passed between the top most layer, and each underlaying ones.

I s'pose cursive fonts might generate too many nodes when expanded. But for simple text, works OK. First attachment, Text at 128 pt., 2nd, 12 pt. Pretty much the same using subtract, top character, or expand.

Try the FFT denoise filter
 

Hi, kateo,
My understanding of SVG syntax is rudimentary. What I know of XML is that "xmlns" is used to disambiguate  terms that might have the same letter set, but mean different things in different vocabularies. I'm guessing that the declaration (xmlns:serif="http://www.serif.com/") is a way to clarify that the term serif, usually part of a letter form, can mean in the file a reference to the Serif corporation. 
Why Canva has a problem w. that, I'm pretty much clueless. Cut it out of the file, and see if Designer has a problem. I've used svg files from various sources, none upon quick review have the line you posted, and they open fine w. Designer. Its only when I mangle the file w. clumsy larger scale editing that I get errors.

You might be better exporting to a PDF file because this is how Affinity Designer opens illustrator and Photoshop files, it accesses the PDF portions of the PSD and Ai files, You can embed fonts in the PDF export and Illustrator will open them. You can also tell the PDF export not to rasterise anything within the file.

Hi, Karina,
I don't do this sort of work much, but I recalled the Daub papers tile seamlessly. Just tried a few, and they did. Drag the asset onto the workspace, and duplicate it over and over to fill the space. 
Then use the menu "File/Edit in photo" In Photo, according to the Daub tutorial, the paper is used as a mask for the brush strokes. The paper texture blends w. the color stroke, adding the light or dark changes from the paper bumps.

This is only my guess, but from using Illustrator quite a bit, not Photoshop, I can't imagine the lines in side "Strong" were not done by hand. The outline of the letters vary in size and shape, but there is not a line everywhere. Look at the bottom of the "S". The main stroke has no inner line, and no part of the embellishments has any. The method hengkidh shows would work to make an expanded stroke that could be subtracted from the letter to show the black layer underneath. But only after being carefully positioned.
I looked up the creator of the "Freedom, Fairness...: illustration, who I suppose is also the maker of the "Strong" pic. It appears he does his work all by hand to start. Pencil, followed by technical pen. He does sell some fonts, but none of those have the dark center lines. I think all his work is basically ink on paper, then carefully digitized and turned to vectors.
Here is the only help I can offer, 2 letters made by hand, but vector from the start.
Fa.afdesign
 

Had to dig around the file system, had them in a folder w. brushes, and not installed. As my parents said many years ago, "You'd forget your head if it wasn't attached to your shoulders..."

To clarify my verbiage, my understanding of the way Designer works is that the vector implies an enclosed area. In ordinary use, one will see at least unclosed fills and stroke lines. 
In my example above, by joining the vector curves, that reduces the problem of Designer's default of closing open paths. As I mentioned, one get's a "snake" that can cut out from the underlying area w/o adding a delimiting line between the ends of each separate curve.

And yet another way....(*could use a little tweaking of course*)
The way I see it is that's there's really just one piece. The funky blue shape in the corner. (not fun in affinity)
Four of those make up the base color donut. That same group is copied and flipped and filled with some simple (base white) shading to get the overlaid features.

Actually just using something as simple as a sheet of A4 copy paper sat or taped down over the tablet can make a world of difference to the tactility when drawing, you get the textured feedback, the sensation of drawing on paper, try it and tell me what you think

Karina,
Assets are collections of objects that one may want to use over any number of projects. They can be all sorts of things. Bitmaps, vector object, symbols. For something like the Daub papers, its easier to just open the Assets studio, and go to them, rather than dig thru the computers file system. That way they are all in a bundle closer at hand.

Or just going to a full 3D app.  Learning to use a 3D app will take some time, but then any number of view angles and lighting direction w. shadings will be very easy. And then, I suppose, posterize it, and vectorize.

Or just going to a full 3D app.  Learning to use a 3D app will take some time, but then any number of view angles and lighting direction w. shadings will be very easy. And then, I suppose, posterize it, and vectorize.

Altho' rather expensive.

Karina,
The papers install as assets in Designer. When I tried out mine last night, I just transferred the file to APh.

The "Daub" tools include papers. While I have the brushes, but I don't seem to have the paper textures.
If you do, look at the DAUB tutorial on how to get something that is more like the natural effect of transparent watercolor on paper.
 

Had to dig around the file system, had them in a folder w. brushes, and not installed. As my parents said many years ago, "You'd forget your head if it wasn't attached to your shoulders..."

This topic was interesting to me, so I spent a few hours searching around. Nothing very good to report. I found just one other image with that letterhead.  It was equally useless for manual or automatic tracing.
Tried to find an old "specimen" book. There were 2 at the internet archive. One from the Cleveland type foundry c. 1890, and another from the American type foundry, c 1910. Neither had anything quite like the older US Patent Office font. A few were close, out of hundreds of examples. But no match.  Check out the "Boston Black" from the Cleveland Type Foundry, pg 221. It may be possible to interpolate the good quality scans of that w. the patent office banner, and make a hand trace. Evidently, the old blackletter style was no longer common by then.
From what I've found at other times, I suppose the example was from a one off design the Patent Office used for a few years. I didn't find today, or see any other time, embellished Caps like those. Any skilled calligrapher was supposed to make embellishments on the fly.
Another possible guide:

Examine the file for clues.
Quick.afdesign

Had to dig around the file system, had them in a folder w. brushes, and not installed. As my parents said many years ago, "You'd forget your head if it wasn't attached to your shoulders..."

Nice colouring, I think the eye nearest needs adjusting it doesn't look quite right, seems too low or needs rotating a bit, possibly more curve to the eyelid on the right side. Could be because the eyes together don't quite fit but something isn't quite right.