@thomasfuchs I have been using Syncthing for years now to replace Dropbox in my life. I'm not sure it's exactly the right solution for you (I don't know how it handles "small teams"), but I was able to get it installed on my QNAP NAS (via a Docker container) for backing up and it works really well.
@thomasfuchs Well, I don't have all your use cases but I replaced Dropbox with NextCloud & I'm perfectly happy with it. Pretty sure it can be set up for various sync options but I don't even use photo sync from my phone so can't speak to that - but integration between Mac, iPhone & Droid has given me zero problems so far (about 1,5 years in).
I don't selfhost BTW; my host is thegood.cloud, but SH is obviously an option with NC.
OwnCloud or NextCloud? Most cloud providers offer this. Synology doesn’t care from which sources you do backup, and you can create an encrypted area on the Synology NAS for secured backups.