You are unlikely to get a mini version when the FP5 comes out and almost all alternate OSes are Android. Google run the AOSP (Android Open Source Project) which many ‘de-googled’ OS’s are based on.
There are some and quite a few Linux, non Android, but the apps and support on them will be very small in comparison.
Best to try and get used to an FP3 or FP4 before you make a decision or any Android phone and try to either install an alternate OS or de-google it.