If it allows you to unlock the bootloader, flashing isn't really that difficult anymore. What really matters is how well the device in question is supported by the custom ROM and the community. If you can find one that's officially supported by lineageOS, for example, they have good documentation and guides to walk you through it. If the ROM is less popular, you might end up in Telegram channels asking for assistance.

The most important thing you can do is research what you're planning to flash. Check XDA, find Telegram, Matrix, or Discord channels (which I know is extremely annoying), look for other people that have attempted to do this and note what issues they seem to have had.

With Xiaomi, you're looking at a two week or more wait to unlock the bootloader. I wouldn't recommend it.

If you are going to spend this much money on a phone to flash, get yourself a pixel and add graphene to it.

If you're only interested in flashing the European version of MIUI/HyperOS then yes, it's very much straightforward and I can see this device is supported.

If you however want to install a custom rom (LineageOS based) then it's bit more complicated.

Xiaomi is one of the most annoying to unlock. you need to create an account with them and wait like two weeks or more.

They changed their bootloader unlock policy. Avoid like the plague.