OP,
No, you're being perfectly reasonable.
(Though definitely investigate/ push for the door to be properly fixed).
(Plus you may be doing a favour for other flat dwellers there also experiencing the same to a lesser degree: my last apartment wasn't right by the communal front door, like yours, but when the street door slammed, it nevertheless reverberated really badly right through the entire [very old] building.
Friends on the ground floor with toddlers had a nightmare, and it was them that got the ball rolling).
It's one thing for babies to learn to sleep through noise, but it's the actual bone-shaking vibration as well as a loud bang that upsets them more.
Especially when repeated over and over.
Your best bet is visual and simple, which sinks in far better with some people.
Maybe combining these 2 images (or similar), and an added "Thank you" beneath? Obviously not an A4-size eyesore...a small version at eye-level will do the job fine.
If you don't feel able to discuss fully with the other residents, which can be tricky when you're new, leave a communal note in front hall mentioning that you're having popping in and out to do some work over the next few days/ week...you hope the other flat dwellers don't mind the temporary sign on door, but you thought it sensible reminding the not to disturb anyone else, by not realising how the banging door noise/ vibrations carry...(NDNs might take the hint).
Nobody in that building took the least offence, when those parents resorted to similar: luckily, we're all rational adults, and didn't all jump on the defensive if something got drawn (politely) to anyone's attention...closing a door more quietly is hardly major inconvenience, FFS, yet can make radical difference to the other person suffering (and prevent simmering resentment leading to all-out NDN wars, let's face it. And in turn gives you leverage if/ when something they're doing might need tackling).
Often tends to be the visitors of the actual tenants, hence permanent reminder useful...leave your "temporary" sign in place until someone else removes it.
Suppose you could try asking whoever's most approachable round for coffee, casually ensuring they experience for themself just how loud it is (coincide the time for peak foot traffic in and out, and just happen to be showing them something in that room etc).
Good luck.