"For certain things it's very, very good (moving a character around on a screen and attacking enemies and collecting things, it's great at that)." -- I think I was lucky in my limited experience with it then, you seem to be much more experienced in gamedev so I am surely not going to argue against your experience with GML + GameMaker, it must have its limitations :)

I was never really interested in making/playing video games but Hotline Miami changed that so I decided to play around with GameMaker and GML and had a lot of fun, didn't have a particular game design in mind so I just made my resume in the form of a platformer, exported it for web, threw it on my website and that was that( https://abhirag.in/interactive_resume/ ) [doesn't work on small screens] I loved the experience and plan to work more with it in future, but will keep your advice in mind and try out Unity too :)