I have three PI’s. Two are headless and controlled via Firefox on Tablet or PC. One is official Pi 7" display and case.
The only issues I have at present with the display/control are :
1] Official display is mounted upside down in case so display contrast is poor. Get better display if turn whole unit upside down!!!
2] When touchscreen screen saver has activated, you need to touch the screen to turn it back on (not surprising) but your first touch is taken as a valid action and acted on. The screen saver add-on needs to have an activation period added that discards screen touches for a configurable time period such that the user does not get unwanted actions due to their initial random screen touches.
As far as user experience, I prefer to use the web interface if I am browsing to find content to play, and only use the touch screen to pause/stop playback if I am in the same room and need to pause quickly.
I do not use a mobile phone, so find the web browser access a bit inconvenient in as far as having to get to the table/PC launch Firefox then find the shortcut to the appropriate target device, then press PAUSE/PLAY can be a bit slow, but I would expect that to be just as slow when using a phone, as a phone is just a small tablet. I also find mobile screens too small to be usable especially if you have to type in or scroll.
I think I would agree that the text/font size on the PLAYING screen could be a bit small if you were reading from a distance, a user selectable font size could be of use, but needs careful consideration as it could impact on other usecases.
If I were to suggest a feature/improvement it would probably be the addition of user selectable font sizes, probably for each of the three major display pages, but this would need very careful testing to ensure it did not cause more user issues than needed. But a larger font may also be of use to visually impaired users.