Fairphones due to having pretty long lasting hardware are common early targets for LineageOS and PostmaketOS devs, so yeah definitely a good choice for longevity.
Google pixels are the best mainstream longevity alternative due to developer adoption in the non-Google Android communities and mobile linux communities. Pixel 1s are still getting updates to latest Lineage Android, though I’m sure it has to be super slow. Graphene only runs on Pixels.
Librem 5 or the PinePhone would probably be your best bets if you want an out of the box mobile gnu/linux.
The Light Phone looks pretty neat and I like the idea of a more minimalistic device (especially with e-paper), but it’s pretty unique hardware and a custom Android that needs jailbreaking to update if the company stops supporting it.
It also looks like the third iteration won’t have an e-paper display, so I’m not sure the beneft of that version will be against a ultra power-saving mode / locked down Android or a mobile Linux on much cheaper hardware.