I guess #lineageos, #openos, #ubuntu and #sailfish are the best options for you.
Personally I recommend Lineage OS, because you can use the F-Droid store (afaik not available on Ubuntu or Sailfish) and it is further along upgrade-wise than Open OS.
Im using /e/ and it runs stable, I used Ubuntu for a while and realy liked it, it did run well and I realy liked it, but you don’t have all the latest Android apps. I tryed Sailfish for a few days, but it didn’t work out for me. When the next OTA is out I’ll probably switch back to UT.
I’ve been using LineageOS on my phone and I’d recommend it, even though I had a couple of issues with it: camera quality completely destroyed compared to the default that came with the original OS, and kinda big deal was that my phone stopped being supported suddently and I’m running it without any updates for months now (hence why I’m here since I’ve been looking for a decent alternative and found about Fairphone).
It seems like my “feeling” was correct, that LineageOS is the best alternative, in a field, with no real alternatives. =/ I had a feeling that the Ubuntu OS was also Ok.
I guess having no absolute reason to leave the Fairphone Open OS, I will just stick with it for now, and table this action for another day. The simple fact of updating required apps (2FA), etc, all make it unnecessary for me at this time.
I have used SFOS as my main OS since 2013 and love it. Useability and privacy are excellent. Just beware, FP implementation has no Android app compatibility, and the stock browser isn’t compatible with lots of sites (e.g. ebay) so it will take some getting used to.
I’m running Ubuntu Touch on my FP2 since April, when I bought it. I really love it. It still has some bugs though, I would still consider it beta, even if I use it as my primary phone.