Lots of caveats there. To preface, most active rom development used to take place on forums, mainly XDA. Nowadays it's on telegram, which is gated unfortunately, so lot's of knowledge get lost.

First, you NEED to be able to unlock your bootloader to be able to install custom ROMs. Here is a source

Back to hardware, recent chinese bought Xiaomi are nearly impossible to unlock bootloader without paying a 3rd party. EU/Global ones can be unlocked, but it's a hassle (waiting 30 days + need to have windows). That makes finding device tree for them hard let alone, finding ROMs.

Oneplus unfortunately is getting worse. Op13 onwards has hardware ARB (which Will brick your device). OP15 onwards has a bit complex way to unlock bootloader.

If you buy it from China, it must be ColorOS version otherwise, you won't be able to unlock the bootloader. Even with COS, you need to install an app to get an unlock token from their server to allow you to unlock the bootloader (this for now, takes 24 hrs max, but this might change and could be like Xiaomi). The Global/EU ones, are fine for now.

Remember you need to have the device tree (basically all the recipes for the hardware of the device to get it to work with other ROMs). Google used to publish this for Pixels, that's why it used to be easy to build ROMs for Pixel. This has changed recently.

So it's getting gloomy for Android devices. I just wish Fairphone would actually follow their "eco" mission, and become the new Pixels. On the other hand Jolla seems to be following up with a no compromise "Linux". Time will tell.

That was actually my first choice I was gonna do as well. I got myself the Nord N200 5g but I unfortunately received the wrong model from Back Market. I returned it and ended up getting this phone for less of a headache to try to find the exact model of a phone in order to install LineageOS. That's my only problem with them.

It's extrmely easy to degoogle xiaomi phones too, but they're a bit more expensive.