christerdk

For macOS see as a free to use one:  CutOut 6 – Kostenlose Vollversion (Mac)
And from the Franzis site:  CutOut 8 Mac note also the Demoversion below there.

So, in the end I solved it by making an oval shape fit the plate, then making a mask from it. From there I "cherry picked" the edges I needed (because the layer was already rasterized, unfortunately), making for a well formed edge of the plate, but still allowing the bread to stick out. 
Curious to hear how those of you that are more experienced would have fixed it, I'm sure I can learn something!

@Dan C I used your proposed process for this one, preserving a copy of the original below to have that serve as the shadow with linear burn. How am I doing? A challenge was that the white balance was slightly off original picture, making the white have a grey tint. To make up for that without loosing too much of the shadow (loaf is slightly lifted over the surface, fyi), I used a Curves adjustment and adjusted the brighter parts. I'll try to fix the white balance at the source next time.
@debraspicher Resizing after upload worked great 👍🏻

Another attempt.
Improved: the aperture is higher, so better edges on the bread to trace. Definitely a lesson learned.
Not yet improved: I don't have a proper stand for this bread, so I had to hold it quite high. Therefore I had no natural shadows to use, so the shadow is fake. 
 
 
 
 

Indeed, that would probably be my preference too - and probably my wallet's preference, too 😆
Apple's trackpads are truly amazing for the everyday use. I produce music as well, and it works really well. In the context of Affinity, I've used the stabilizer to make up for lack of precision. Since I'm a hobbyist I'll probably go down the mouse route first, so see if that is enough for now. 
Not sure if it's allowed to post here, but here is my IG sourdough page (let me know if not, I'll remove). As you can see, I was taking photos for a while in natural light outside. But after a move this was not possible anymore, so I shot pictures in the kitchen, now with lovely lovely florescent light. And after seeing some of those picture, a friend of my mine commented that "it's time to get a lightbox"... 😄 

There's a much easier way to accomplish what you're doing using the Channels. Go to the Tutorials Section of the forums, scroll down to the Advanced section and you'll find links to Channels and Channels: Selections.
The Legacy Tutorials are still found in the Tutorials Section. Scroll down to the Channels Section, there are links to 6 tutorials explaining them.
Channels Channels for Alpha Masking Channels for Creating/Storing Selections Editing Single Channels Channels Greyscale Blending Channel Packing I provided links to the specific tutorials that I think should help you understand and use them. Once you learn them, you'll see how powerful they are in Affinity Photo.

Yes, position any object above the master tile of the Pattern and then use Merge Down command to incorporate into the Pattern.

This is a great macro, worked well with my goal, to superimpose something on a dollar bill 👍🏻

I'm only working on one page here at the moment, so didn't see that perspective, but that totally makes sense. Thank you 🙏🏻

Hi @iconoclast!
Thank you for your input. Shift-Cmd-W didn't yield any change. But I went all in on trying permutation of your suggestion and...
For Mac it's Ctrl-W
Just writing it out, in case anyone else is experiencing this.

Btw, @iconoclast, now that I have you here. Do you have any idea why only parts of the picture is shown in my original picture. It's like there's som sort of ratio involved that I'm not sure where stems from...

There are a bunch of T-Shirt design & mockup tutorials & videos on the net, just do a Google search after that topic and you will find many on Youtube & Co.
affinity designer t-shirt design affinity designer t-shirt mockup affinity photo t-shirt mockup ... and so on ...

Good to know; for anyone else who is wondering…
Lock Children is in the Context Toolbar when you are using the Move Tool, and only when the selected layer has children.
Sometimes that toolbar is too long to be displayed on-screen so some of the functionality is put on a ‘jump-list’ which can be accessed via a little chevron icon at the far right – see attached image.

I life hacked it by removing the blocking div, could enter the CC card information and purchase. 
I'm on Monterey, Chome 101.0.4951.64, in case you want to follow up. Offer to send video still stands.