Agent based backup server?

So, I have a VPS running some stuff, local proxmox-setup running something and then the 'normal' computers (laptop mostly) which I'd like to get a bit better backup solution for.

Proxmox VMs are taken care of by proxmox backup server and hetzner storagebox + nas at the separated garage, so they are decently protected against hardware failures. Workstations keep important files synced to nextcloud and the VPS has it's own nightly snapshots at the provider, so there's some reundancy in the system already. However, as we all know, two is one and one is none with backups, so I'd like to add a separate backup server in the mix.

As there's devices which are not online all the time I'm leaning towards an agent-based solution where devices push their data to the backup server instead of server pulling the data in. Also as I have some spare capacity I'd like to have an option to offer backup storage for friends as well where agent-based solution is a practically requirement.

But now the difficult thing is to decide software for it. Veeam offers something for hobbyists, but I'd rather have more open solution. Bacula seems promising, but based on a quick read it doesn't seem to be that simple to set up. Amanda looked good too, but that seems to be more or less abandoned project. Borg Backup would fill my own needs, but as friends tend to have either Windows or OSX that's not quite what I'm after.

Any suggestions on what route I should take?