The phone is crashing because you do something it wasn't designed to be used for. They won't upgrade your phone for free, that's for sure.
If you can't live without games, I suggest you end your contract early, that will cost you about 100$ (2y contract, you said it ends June 2020, phone is 200$, so it's 100) and then get a phone. The Galaxy A70 is coming very soon and it will be a much better gaming choice.
That's a really lame rationalization - There's plenty of apps on the Play Store & Galaxy Store that aren't compatible with every device.... Just deal with the fact that your J3 isn't designed to be used for gaming... 🙄
And if tobacco and antifreeze wasn't appropriate for human consumption it wouldn't be sold in stores...
If you have access to something, it doesn't mean you should use it. The stores you mentioned also include apps like e-mail, calendar, calculator,Internet browser, banking... and unfortunately sociable networse apps. So no, you don't have access to games because your device can run them in a satisfactory manner.