I like your first idea better. Why does your wife hate fun?

If you don't need a good lock, any smart lock will do. Just watch the videos by the Lockpicking Lawyer on YT. Those "smart" locks are rather horrible. For most, you don't even need lock picks.

honestly?

Just put a small locking mechanism on the door high up. Or just put a normal lock on it, put a nail in the wall up high where they don't reach it and hang the key on that nail.

Smartlock seems kind of overkill for this.

I've used this handle before that sounds like it would suit your needs. Of course it also requires Zigbee or Z-wave connected to your HA setup if you want to log locking/unlocking in HA.

Are these miscreants two-legged or four-legged?

They make maglocks that you could put in the corner of the doorframe. Then wire it to an esp32. Esphome to connect it and home assistant to control.

I use one for a liquor cupboard that auto locks if my wife and I both leave the house. (well our phones anyway)

Any smart lock should work on an interior door, you may have to drill a deadbolt but its the simplest solution. Switchbot and U-tec both make smart locks with fingerprints and excellent Home Assistant integration. Ubiquiti also sells magnetic latches you could install for access control if you're in their ecosystem.

If you can give up on Home Assistant integration/smart features, however, there are oodles of push-button and even fingerprint door (without other connectivity features) knobs out there. Most of them are terrible quality and not secure but that doesn't really matter to you and they are cheap.