Answering your questions in order:
- On mine I use NFS and SMB which covers most possible clients.
- I use an ssh bastion that I expose via Tailscale to connect to mine remotely. So a VPN but it's wireguard based so it's not too intrusive. I have a gig up, though, YMMV.
- My NAS has 28TB of space. I'm still working on backup strategy. So far it just has my Dropbox and some ephemera I don't care about losing on it.
- Regarding other services: I use Dropbox pretty extensively but these days 2TB just isn't very much. Plus it gets cranky because I have more than 500,000 files in it.
This is my personal setup but I think it's a bit different for everyone.