for what it's worth, I wanted to share, that I am getting nice results from using python's kivy gui framework, together with nimpy. (compiling the Nim code to a library). It has cross platform capabilities, and you can target android as well. (don't know how to do a macos, or ios without a hardware mac - I guess even in virtualbox you need macos iso's?)
They have a build tool - buildozer, though slow and quirky (buggy?), it seems to have everything needed to build an apk, it downloads whatever is missing on your system etc.
It is my first travel to android land, so if there is a much more streamlined process from experienced devs, I would love to hear that.
Another (stray) thought I have is that with python moving to types, there are also tools to infer types (for example pyre-check), it may be easier to port important python libs to Nim.