I’ve just installed some Amiga emulators for Android from Google Play store and at least two of them (Uae4All2 and Uae4Arm) are also SDL2-based. Both have an SDL2-based GUI (and Uae4Arm GUI is identical as in Amiberry) and their SDL GUIs are working OK, i.e. good screen mode, autorotating and can be normally controlled with touch.
The emulation itself also has the right screen mode (autorotates, fills the display in landscape) and even touch works ALMOST fine - i.e. the pointer on Amiga Workbench can be controlled via touch screen, it’s just that there is a large discrepancy when it comes to coordinates - the pointer moves much faster than the finger, so I guess it needs some tweaking. But other than that, even click and double click work OK to the point that even in Workbench you can click icons, open folders, etc.
The following are screenshots from Uae4All and Uae4Arm for Android running on the 10 III.
So maybe both being SDL based and working almost OK, they could be helpful in tweaking Amiberry GUI and touch control?