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.
As others have mentioned, there are no good BifL options. Based on what I gather from your post, your best option is probably getting 2nd hand devices and following behind by a few years. You can probably keep a 3-year-old device for 7 or 8 years (which is ages in the smartphone world), then “upgrade” to another 3-year-old device at that time.
For this, I’d recommend something popular like a Pixel. They have a number of options for alternative OSes (Graphene and LineageOS are both good options) and they’ve done well for me as long-term use phones.
I’ve bought my last couple phones on Swappa, and I’ve had no issues with any of them. Sold one on there too, and they’re pretty vigilant (they manually review posts before they can go public).