Current FOSS supported smartphones are mostly not really compatible with BIFL
Firstly, Rooting/ flashing non-manufacturer firmware voids your warranty. A phone without manufacturer support is going to struggle to be BIFL. If you’re able to flash the original ROM back on that protects you some, but if a failure leaves you unable to flash firmware then you’re SOL.
Also, the FOSS OS might be solid but many ports to specific phones are enabled by only a couple of developers doing it on a voluntary basis. You might well find multiple FOSS OS are being maintained by a single person who is really into keeping their phone compatible with multiple FOSS projects. Unless you’re going to be that person, you would want to check that’s not the case, because if that person upgrades you may find yourself without support which again is not BIFL.
The closest you’re going to get is probably something like the Fairphone. They’re designed to be long-term repairable, ethically produced and running FOSS. But they are also far from the cutting edge. Pick your poison.
KZ ES4 were recommended by DankPods as great cheap wired headphones, in the UK they’re £20 and a replacement cable is £10. They’re 3.5mm so you’d also need a USB-C dongle. Not exactly BIFL but I liked them, although I daily drive AirPods now because the convenience factor is just so much higher.