So far what I found:

My router has an option to block internet access by MAC address, which is even better than by IP.

LAN mode works via wi-fi, so no cables or flash drives needed.

Also Orca slicer is almost the same as Bambu studio, even supports printer's camera. Feels even snappier without an account and integration with "store" or whatever that online stuff is called.

Got more interested in trying out VPN now, so I probably could observe prints from a laptop in any location.

Not as bad as I feared, not as good as it could be.

Easy mode: don’t connect it to internet

Slightly harder: add exceptions for bambus servers in your routers firewall so that requests to that domain are blocked

Dependent on hardware that you own and less likely to exist on basic routers: create a virtual lan for your 3d printer that does not have internet access but has intranet access, allowing you to connect between computers but not to the web (my preferred solution)