Do note that although custom ROMs helps provide OS updates, it does not help with any firmware updates to your phone parts as those are vendor released. Once they stop providing those updates, it is no longer secure
I have a OnePlus 3T (2016) that is running Android 11 custom ROM. I use it mainly for some games and browsing. I would never use it as my main phone or use banking apps there though. Don't want to risk all of my data on there.
That's why GrapheneOS only supports phones that are still officially updated.