If you want to switch specifically for free software, make sure that the phone you're buying is bootloader-unlockable. The Pixel phones (not from a carrier, but unlocked) are good options.
If not, you will simply be locked into Google's ecosystem (along with whatever OEM, such as Samsung's), which isn't much better than Apple's.
As for ROMs, I'd recommend either GrapheneOS or DivestOS. Both are free of all Google services by default, and are as FOSS as Android allows for in the modern age.
To deregister iMessage, visit this site: https://selfsolve.apple.com/deregister-imessage/
In the future, I'd look into Linux phones, but as of right now, they are not usable for daily driving IMO. You can also test Linux mobile on most modern Android phones using Halium with a distro like Droidian.
Also check out privacyguides.org for alternatives to proprietary apps/services.