OLED alone even without HDR adds a noticeable difference in contrast ratio. Meaning blacks look blacker even when right next to bright whites. HDR improves that, provided you have HDR content to enjoy.

An issue with some (much) older OLEDs was burn in, but at least in my experience, with more modern displays that seems to be much less of an issue. A lot of displays have a burn in reduction feature on board that seems to generally work well and the actual LEDs have gotten more durable as the tech has advanced.

I have an OLED display hooked up to an old rpi running my homeassistant control panel. It's been displaying an essentially static image for nearly two years without any burn in.

Personally, I'd recommend an OLED monitor. If you can afford it, go for high resolution and high refresh rate. If you primarily watch video prioritize resolution, if you primarily game prioritize refresh rate. Though you may have issues going over 120Hz on Linux.

As for your DE, Mint should support KDE Plasma and you should be able install it like any other package. Might be worth looking up a guide for that. However, I won't recommend against switching to Fedora. It's what I use and I haven't had any notable issues and their documentation seems pretty solid.