Speaking of filesystems- is there one that allows me to add tags? Or alternate streams (like NTFS does), so I can add my own tagging system on top?
What you want is to store the tag metadata heap in an easily indexed structure (i.e: a search tree.) That heap then just contains pointers into content addressable storage. You can kind of build such a thing on top of existing filesystems which support hardlinks, however I'd advise against that for the metadata itself. Once you have the power of tagged content a natural evolution is querying that data using set operators. You will inevitably re-invent (some approximation of) SQL once you arrive at this discovery.
But not tags themselves- so I'll still like a file-system that supports tags natively.
On a NVMe drive rebuilding the search index shouldn't be too costly, even on hundred thousands of files.
I think there were also some ioctl additions for btrfs that let you sequentially walk the filesystem by extents or inodes instead of by directory tree, which makes indexing a lot easier.