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)

As an “outside observer”, I think maybe you’re not seeing (what I believe is) the other guys viewpoint: What you are bringing up (photoshop has been possible already) is a core part of what he said from the start, and his point builds on top of that. So obviously he already knows it, and arguing about it disregards that his line of argumentation builds upon the basis we all agreed upon to be true until you brought it up as … contrarian? To his point. doesn’t seem like “old man yells at cloud” energy, more like “Uhm, achtually”