As for aspect ratio, “soon” there will be an option for it, but I am waiting for Ebitengine 2.5 to stabilize first, as the code for this is in a way that differs from the Ebitengine 2.4 way of doing this so I put this on my 2.5 branch.
In the place where the desktop setting has a fullscreen option, the mobile version will have a stretch/letterbox option.