from my experience, these are the steps you should follow:
1. turn off the device
2. on a pc with network card disconnected or without it, install a program to format devices
3. wait 6 months for law enforcement forget about the theft
4. once the 6 months have passed, reformat the device again
5. wait one or two weeks
6. once the weeks have passed, install a ROM without Google services on the device (the best option is an AOSP ROM ), although you can also try a Linux distribution, also install a custom recovery.
These are some tips, but remember that stealing is illegal :))