I wouldn’t choose a custom rom on the sole basis of anti-theft. My ½-baked suggestion was simply disable the playstore framework (so it’s present but just dead code) and installing an app on the side.

Anyway, I have no interest in anti-theft bricking myself. I don’t envision ever having a phone where i would care about the hardware and would not likely spend more than $50 on a phone. Exceptionally I could one day get a Fairphone. But remote bricking does not tempt me. Making the phone a brick more quickly gets the phone into a landfill as it becomes useless for everyone.

It’s worth noting why phones get stolen. Even cheap phones are getting stolen. It’s not for the hardware. It’s because SIM registration makes it hard for criminals to get anonymous burner chips. So they steal phones just for GSM chips that are registered to someone else.