After looking into the OsmocomBB project more, I’ve realised that all hope of an open-source baseband processor is hopeless.
They’ve managed to reverse-engineer a GSM baseband processor, which somewhat works on a select few 10+ year old phones and only via getting some leaked specs. And from what I’ve heard 3G and LTE are also an order of magnitude more difficult to reverse engineer.
So let’s instead focus on the lower hanging fruit and the actual solution The Tor Project realised in the previously quoted post - hardware baseband isolation.
Their immediately proposed solution was having a tablet + a separate portable router with your 4G SIM card but we can actually have this all in one phone.
The Neo900 is a smartphone which is doing just that.
So if they’re able to do it and have a modular phone, I don’t see why Fairphone wouldn’t aim to do this also.