If you are looking for a hardened phone, I would consider trying GrapheneOS for a bit, see if it does what you are looking for. Uses SELinux and a seccomp-bpf policy for app sandboxing, as well as runs a hardened kernel with a hardened memory alloc. Great isolation approach, too, so that you can run apps on a 'completely different phone,' so to speak -- think of the isolation like a small version of the OS that can keep apps entirely separate. Finally, if desired (and needed for certain apps), you can sandbox all Google services so that they don't have direct access. It's is a different approach to, say, microG.
GrapheneOS is all about hardening. Security is solid.
VPN wise, Mullvad wireguard servers are also solid. You can do multihops, which help you obsfucate traffic to degree. They have also been playing around with packet shaping (if you use their app directly).
Sim cards can be swapped out if use a VoIP service like jmp.chat.
I love it!