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.

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