I don't have a full answer to snapshots right now, but I can confirm Nextcloud has VFS support on Windows. I've been working on a project to move myself over to it from Syno drive. Client wise, the two have fairly similar features with one exception - Nextcloud generates one Explorer sidebar object per connection, which I think Synology handles as shortcuts in the one directory. If prefer if NC did the later or allowed me to choose, but I'm happier with what I got for now.
As for the snapshotting, you should be able to snapshot the underlying FS/DB at the same time, but I haven't poked deeply at that. Files I believe are plain (I will disassemble my nextcloud server to confirm this tonight and update my comment), but some do preserve version history so I want to be sure before I give you final confirmation. The Nextcloud root data directory is broken up by internal user ID, which is an immutable field (you cannot change your username even in LDAP), probably because of this filesystem.
One thing that may interest you is the external storage feature, which I've been working on migrating a large data set I have to:
can be configured per-user or system-wide
password can be per-user, system-wide, or re-use the login password on the fly
data is stored raw on an external file server - supports a bunch of protocols, off hand SMB, S3, WebDAV, FTP
shows up as a normal-ish folder in the base user folder
can template names, such as including your username as part of the share name
Nextcloud does not independently contribute versioning data to the backend file server, so the only version control is what your backing server natively implements
I use LDAP user auth to my nextcloud, with two external shares to my NAS using a pass-through session password (the NAS is AD joined to the same domain as Nextcloud uses for LDAPS). I don't know if/how the "store password in database" option is encrypted, but if anyone knows I would be curious, because using session passwords prevents the user from sharing the folder to at least a federated destination (I tried with my friend's NC server, haven't tried with a local user yet but I assume the same limitations apply). If that's your vibe, then this is a feature XD.
One of my two external storage mounts is a "common" share with multiple users accessing the same directory, and the second share is \\nas.example.com\home\nextcloud. Internally, these I believe is handled by PHP spawning smbclient subprocesses, so if you have lots of remote files and don't want to nuke your Nextcloud, you will probably need to increase the PHP child limits (that too me too long to solve lol)
That funny sub-mount name above handles an edge case where Nextcloud/DAV can't handle directories with certain characters - notably the # that Synology uses to expose their #recycle and #snapshot structures. This means that remote mount to SMB has a limitation at the moment where you can't mount the base share of a Synology NAS that has this feature enabled. I tried a server-side Nextcloud plugin to try to filter this out before it exposed to DAV, but it was glitchy. Unsure if this was because I just had too many files for it to handle thanks to the way Synology snapshots are exposed or if it actually was something else - either way I worked around the problem for now by not ever mounting a base share of my Synology NAS. Other snapshot exposure methods may be affected - I have a ZFS TrueNAS Core, so maybe I'll throw that at it and see if I can break Nextcloud again :P
Edit addon: OP just so I answer your real question when I get to this again this evening - when you said that Nextcloud might not meet your needs, was your concern specifically the server-side data format? I assume from the rest of your questions that you're concerned with data resilience and the ability to get your data back without any vendor tools - that it will just be there when you need it.