I like Affinity Photo but it is an absolute hell using it on a touch-only basis. I use a Surface Pro 7 and there are glaring problems with touch screen usability that bring me to a boiling frustration every time I use Photo for more than a few minutes.
We need touch controls in the Toolbar for common functions. Undo/redo, deselect, incremental rotation, etc. need Toolbar controls, whether added by default or as an option under the Customize Toolbar window. The fact that I have to sit and repeatedly do Edit > Undo using a pen or my finger, because the simplest thing like an undo button is missing from the UI, is insane. Especially when I make a sketch and then decide to change something, and need to hit undo about 40 times. Immense oversight.
The option to disable and hide the canvas viewport's scrollbars would be invaluable. Touch misfires are bad enough when they cause the canvas to jitter or cause a tool change unexpectedly, but when the slightest "tap" of a nonexistent finger hits the scrollbars, your entire canvas gets thrown to the side instantaneously. Very disorienting, and happens at least once a day in my experience. I never ever use them since I have both a touch screen and a gesture capable trackpad, so they ONLY mess things up. An off button would be very nice.
It would also be nice to be able to tell Photo to interpret a single-finger drag (assuming you have touch for gestures only turned on) as a pan canvas command, and a two finger gesture then could always be considered zooming. The gesture interpretation between two finger pinch and two finger drag seems to be really finicky and ends up being very clunky.
Finally, though it might be a very niche thing, having a Canvas Lock toggle you could add to the Toolbar would be nice. That way one could lock the canvas while doing intensive inking, to prevent possible jitter due to a palm rejection misfire, and manually unlock it when intending to pan and zoom around the canvas.