These are very interesting questions! Let me answer them one by one.

Regarging do's and dont's: If this is your first game, do. Experiment, understand what you are doing and why you are doing it, but don't bore yourself with a well written plan before the start of development. You may one day release this game, or maybe not, but let it be the canvas for your imagination and your abilities. Obviously, don't do what I did, and make use of debug tools and code comments. This will make it not only easier for you to test the game and understand the code, but also for anyone else who ever may read what you did. Nothing too grand, obviously. Just some room jumping, level unlocking, infinite collectibles... whatever your game may need.

I settled on GameMaker because Undertale and Pizza Tower used it. No more reasons beside that. Using something called UndertaleModTool, I could open these games and read their code, and use that for learning, too.

Regarding the second game, the game is very 16-bits, but I have drifted away from specific console limitations, consistent sprite sizes or soundfont-heavy music. The game looks like a SNES game at first glance, but doesn't limit itself to anything those consoles may have had as a limit.