acordeonista

In the beginning for sure.
It took me literally years until I was ready to switch from PS CS5.5 to Affinity Photo.
Meanwhile, once I understood the workflows – and the flaws – of Photo, using Photoshop is a pain…  

I'm trying and will keep trying. I'm illustrator + photographer so it is a myriad of small, quick gestures of what I've been used to for many years... I'm so spoiled... we'll see! Thanks and cheers, loukash

@loukash Thanks a lot for your interesting help!
It's funny how you get used to do small operations thousands of times, year after year in photoshop, and when you change application any small variation in the operation is a pain...
¡Gracias! 

If I understand it correctly, in "Affinity speak" this is called "Crop to opaque".
For some weird reason it's not generally available for the Crop tool, but it is built into the Panorama mode.
So, after experimenting a bit, here I found an admittedly slightly awkward but manageable workaround:
Rotate your image as you see fit, be it with the Crop/Straighten tool or with the Move tool Document > Clip Canvas (optional, but can be helpful) Export as PNG or TIFF with transparent matte File > New Panorama Add the exported image twice to the list (a panorama requires at least two images) Stitch Panorama > OK (since both images are absolutely identical, they will simply merge in place without any visible change) now you're in Panorama mode: Crop tool > Crop to opaque in the context toolbar Apply Again, don't ask me why "Crop to opaque" isn't available as a standard crop feature… Although I vaguely remember that it has been requested years ago.  
This is another weirdness of the Affinity suite. You should adjust Preferences > User Interface > Decimal Places to non-integer values. In fact, for my most used units (px, pt, mm, °) I'm setting it to 6 decimal places. The issue here being that these are just the display settings for the unit fields, i.e. the Transform panel doesn't actually round up or down the value, it just doesn't show that there are decimal places. 

Forgot to mention
Once developed and back in the Photo persona the Transform panel will allow you to use 0.1° increments in the Rotation field by using CTRL plus the arrow keys
(Windows)

The other alternative is to Inpaint the "alpha zones" which is easy to do and can also be automated in a macro

Unfortunately, I do not think there is any way to assign a keyboard shortcut to open that window.

But you can keep the panel open while working, no need to close it (if you've got space enough on you screen), or use the Colors panel to choose colors.