Use GrapheneOS, it’s a completely independent non-profit mobile security research project that has probably created the most secure mobile operating system out there. GrapheneOS doesn’t use any Google services by default, and it makes fundamental security improvements to the entire Android operating system stack. It also doesn’t have an official app store that comes with system privileges, so technically every app installation on GrapheneOS is ‘sideloading’. In my opinion, it’s by far the best option for a mobile OS.