Depending on how old the phone is, this is less of an issue than you or others here think it is. Starting a few years ago, android at least can't simply be wiped and set back up from scratch. Manufacturers/Android did this in response to the increase in phone thefts. Iirc, this had a significant impact on the number of phones being stolen as an opportunity theft. If someone targets you, that's a different story.

As others have said. Make sure encryption is turned on and that it's setup with a pin. If you really are concerned then make sure the time out to lock is set to as low as possible and never have it unlocked if it's not in your hands.

A Faraday cage is a pretty low tech and ubiquitous way to keep anything offline regardless.
They come in convenient travel bag formats these days, but an old metal can will do.

That’s a possibility, but if it’s locked they won’t be able to log into it to access your data or factory reset it, which will make it essentially worthless to them.

Gluing the SIM card tray will add to that worthlessness but will also prevent you from being able to legitimately sell, trade or give away the phone.