I’ve worked with GUIs on python for a couple of years, we used PyQt, which is a python wrapper for Qt which is a C++ library for GUIs. It’s fairly straightforward and easy to get something up on the screen in no time.
However from parts of your comment it seems you want to implement your own graphics library, and that is a lot harder to do.
Also you mentioned legacy hardware, not sure how legacy it would be. Python should run on most things people would call legacy nowadays, but there’s definitely an overhead that could be felt if you’re trying to run this on an embebed system or a REALLY old (as in 90s/00s era) computer.
You also mentioned mobile, I don’t think PyQt can be compiled to mobile easily, nor do I think you should even if you manage to (been there, done that, not a happy time). Desktop and Mobile GUIs are very different, realistically if you want something that works well on both mobile and desktop with the same codebase the easiest approach is web UI.
Ah, didn’t knew that french also did the gn as ni, that makes a bit more sense, I found it weird that a French person was using an Italian phonem when writing English and was wondering if maybe there was a place with a similar name but google didn’t seemed to think so.