I have a long background building, in architecture, in multi-family architecture, too.
So, I've built a few apartment buildings. Stood up some sample sheetrook walls out in the boonies to see what shoots through and how much. Turns out a very long distance within typical apartments is 21 feet--7 yards. 10 to 12 feet is far more common.
Turns out bullets (and shot) will sail through the very empty spaces in stud-frame construction with ease. There are 4 1.5" studs in 48" or wall--14 1/2" of empty space between those 16" O.C. studs, so, any given square foot is a void.
While the encapsulated shot rounds are somee what slowed by gypsum wall board, they have a problem of being ludicrously expensive. And, really, you need to practice with the round you use, as in dire straights is a bad time to discover that those glazers or magsafes shoot a inch or two (or 6) differently than the rounds you practice with.
Now, one option that has not been brought up in shotguns is a 20 gauge auto-loaderr.
For a handgun, what you really need is a .455 Webley top break revolver. No "handing" bias for reloading, and a big, slow bullet at mild velocity. Sadly, that's not a practical answer.
Which brings us back to where we've been trending--a left-hand 1911, or perhaps something striker-fired in .45gap
As to flimsy apartment door security--that's the nature of the beast.. They generally are loathe to allow any tenant to make substantive changes to their buildings. Occasionally, you can "get away" with improvements like replacing hinge screws with some 3 1/2" screws that will grab the studs and not just the pre-hung door frame.