It doesn’t sound like you’re trying to do more than what the phone is capable of. To me, this sounds more like a hardware issue with the display or SoC, probably a bit of both.
If you don’t manage to find a solution, feel free to #contactsupport and make sure to mention what you’ve already tried. Unfortunately Dec 2019 is outside the warranty period so if you want to send your phone in for repair, it will be at your cost.
I have a new issue recently. Charging doesn’t seem to work correctly.
I need to have the USB cable in a very specific angle or the phone refuses to charge or sometimes only charge slowly and stop charging after a while and then I have to try to find the new angle. And, more annoyingly, it sometimes just restarts when I plug it into a charger.
This happens with any USB cable and charger, even those who have worked flawlessly for years with the phone.