I have had 3 Xiaomi phones over the years. Their proprietary bootloader-unlocker tool has always taken a good day or two of work to get the phone unlocked when I don't have adb tools /drivers installed from the get-go. Their utility gives me failures/errors/denials/"your social credit is too low" (i don't live in/near China) dozens and dozens of times before it finally decides to unlock my phone for me. I'm pretty sure my next phone won't be a Xiaomi, though it's hard to find sanely priced non-Chinese phones with good ROM coverage these days.

Yep, here's the link to the LineageOS device list with installation instructions. https://wiki.lineageos.org/devices/#xiaomi

Unfortunately Google is making it much harder to run ROMs now due to the new Safety-Net bootloader checks. You'll no longer to be able to use many bank apps (or even the McDonalds app!).

You can never be sure what's hiding in the hardware, if you already don't trust the software.

Yeah, that's what I wanted to do but the power button doesn't work anymore so if I turn it off, there's no way to bring it back to life.