I don't have extensive knowledge off the top of my head. I usually take a look only when existing device dies. See GrapheneOS devices list and a bit further down the page there is a table of expected EOL dates. Most other similar ROMs have the same list. LineageOS being the exception with much wider support. I beleive that some compromises have been made to allow that but can't elaborate more than that. There are zillions of nerds debating online about it. Same caveat about bootloader locking applies in any scenario.
IME the best thing is to make sure the device can be returned for any reason, and plan to attempt install ROM asap upon arrival so you have enough time to send it back if unsuitable.