Hardware far outlasts software in the smartphone world, due to aggressive chronic designed obsolescence by market abusing monopolies. So I will never buy a new smartphone - don’t want to feed those scumbags. I am however willing to buy used smartphones on the 2nd-hand market if they can be liberated. Of course it’s still only marginally BifL even if you don’t have demanding needs.
Has anyone gone down this path? My temptation is to find a phone that is simultaneously supported by 2 or 3 different FOSS OS projects. So if it falls out of maintence on one platform it’s not the end. The Postmarket OS (pmOS) page has a full list and a short list. The short list apparently covers devices that are actively maintained and up to date, which are also listed here. Then phones on that shortlist can be cross-referenced with the LineageOS list or the Sailfish list.
So many FOSS phone platforms seem to come and go I’ve not kept up on it. What others are worth considering? It looks like the Replicant device list hasn’t changed much.
What you are tying to achieve is literally the goal of PostmarketOS. For supported devices, PMOS should continue working for a very ling time when compared to more hacky custom roms. I use a Oneplus 6 and have used both divestOS and PostmarketOS on it and it is well supported by both.