I never got to try it, but they are doing exactly what you want it seems. I don't know how much of their backend they have open-sourced, though. I definitely need to research this more.
They do have a matrix channel and a fediverse presence, so they already look quite alright from the start, and the project is very interesting.
A Fairphone 3/4 might be your solution, definitely check it out
They said they wanted to support it for more than 5 years but the industry won't allow it since they stop distributing the components after a few years, but they are gonna support it as far as they can AFAIK.
As for the MNT Reform, the only thing I'm not sure is open is the actual processor firmware, but the schematics for its usage are available and even the Wifi firmware is open, so there remains the problem of actually verifying the hardware you get is actually the hardware you ordered, but that is a bit more complicated I think.
Seems like the website is dead now, RIP