There's no meaningful, usable formally-proven FOSS OS. Being FOSS isn't enough.
Personally I just gave up trying to maintain OpenWRT/whatever on the embedded ARM dumpster fire, and went back to using a generic Linux distro (NixOS) on amd64 machines for both router and APs (with appropriate minipcie wifi cards).
An alternative is OPNsense, a minimal FreeBSD and Deciso supports an enterprise distribution-quality of it affordable by mere mortals.
This is by design. Those who control the past (network traffic), control the future (network traffic).