I'd add:
zerotier + rclone sftp/scp mount w/ vfs cache? I haven't tried using vfs cache with anything other than a cloud mount but it may be worth looking at. rclone mounts work just as well as sshfs; zerotier eliminates network issues
Zerotier + sshfs is something I use consistently in similar situations to yours - and yes, zerotier is similar to a vpn. Using it for a constant network connection makes it less critical to have everything mirrored locally. . . But I guess this doesn't solve your speed issue.
I"m not an expert in rclone. I use it for connecting to various cloud drives and have occasionally used it for an alternative to sshfs. I"ve used vfs-cache for cloud syncs but not quite in the manner you are trying. I do see there is a vfs cache read-ahead option that might he|p? Agreed on the documentation, sometimes their forum helps.
Autocad. This is the main (only?) reason I continue to use Windows. Professional 2d cad for architectural drafting has been lacking in Linux for a long time. There are a few commercial alternatives, Bricscad being the big one, but due to (cheap) grandfathered licensing cost for Autocad, I"ve been unable to push for a purchase. Qcad (professional) was another option I looked at but, despite being a good program at an amazing cost, had enough differences in work flow that I couldn't find a good way to integrate it into a shared workflow.
Every once in a while I switch to Linux and either run a W10 vm or RDP just to work around the issue but, inevitably, get frustrated with performance. Freecad and Blender both seem to be working on the problem -but- from a BIM perspective, not detailed drafting . . .