Some of these you’re already doing, but writing a complete* list. *almost garuanteed not to be complete, suggestions welcome

1. Have everything behind the same reverse proxy, so that you have only one endpoint to worry about. Run it through ssllabs or similar to check your config.
2. On your reverse proxy, add one or more layers of authentication if possible. Many possibilities here: If one app supports client certificates, while another has limited capabilities, you could probably tie together something where IPs are whitelisted to the ither services based on that certificate auth.
3. Geoblock all countries you won’t be accessing from
4. crowdsec is pretty nice, this detects/blocks threats. kinda like fail2ban but on steroids.
5. if you use one of those 5$/month VPSes, with a VPN tunnel to your backend services, that adds one layer of “if it’s compromised, they’re not in your house”.

lastly consider if these things need to be publically avilable at all. I’m happy with 95% of my services only being available through Tailscale (mesh VPN, paid service with good enough free tier, open source+free alternatives available), and I’ve got tailscale on all my devices

I don’t understand what you mean with the content disappearing when you mount the virtiofs on the guest - isn’t the mount empty when bound, untill the guest populates it?

Can you share what sync client+guest os you are using? if the client does “advanced” features like files on demand, then it might clash with virtiofs - this is where the details of which client/OS could be relevant, does it require local storage or support remote?

If guest os is windows, samba share it to the host. if guest os is linux, nfs will probably do. In both cases I would host the share on the client, unless the client specifically supports remote storage.

podman/docker seems to be the proper tool for you here, but a VM with the samba/nfs approach could be less hassle and less complicated, but somewhat bloaty. containers require some more tailoring but in theory is the right way to go.

Keep in mind that a screwup could be interpreted by the sync client as mass-deletes, so backups are important (as a rule of thumb, it always is, but especially for cloud hosted storage)