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).