unlock the bootloader and flash a custom ROM like LineageOS or GrapheneOS. If there is none, root it and rip out Play Services and dependents by force (careful about boot loops with Play Sore itself).
install a trusted app store (F-Droid/classic) and install Firefox or a fork and there uBlock Origin. Or at least a browser like Cromite or Vanadium.
Install replacements for missing dependents like phone caller (fossify to the rescue).
Android 12 or later:
Install replacements for missing dependents like phone caller (fossify to the rescue).