I am big on define-the-threat and went with a cage for my house. Layered, a concrete (tiny) room the stuff is in, then a cage inside that for firearms. All Simplex locks. Suburban, ISO-1, we appreciate the nosy neighbors, we have a low risk of theft, a near zero risk of someone using heavy tools etc, and whole-structure-loss fire risk is low.
(Important documents are in a legit commercial spec fire cabinet, not a consumer grade one).
1000% go with mechanical. Ideally electronics would work, but I have worked (professionally) with embedded systems plenty enough to know its not even worth hoping anymore they will work, as much as e.g. biometrics would help speed up access.
Find a local dealer instead of mail order I tend to say. Here in KC there's one reportedly good where the Bushnell outlet was (they moved next door) but even a trustworthy locksmith may be able to help, whether safe or vault. Keep the local info, they may buy it back (some deal in used ones) or help move it when you move out if you no longer need it.
Think about a vault instead. A secure room, not a box you have to put somewhere. Can be more space efficient, and can be fairly inexpensive as you just make sure the outer walls of the room get suitably reinforced, and then install a very strong and securely locked door to the room (closet etc). Best installs I have seen for new construction are like this, whole side of the basement as a sealed off room etc but possible to do in existing construction. You can even upgrade slowly, start with just securing the door then move to secure the walls, make organizers, etc.