Still testing, but I’m currently mounting my remote storage via sshfs-win. There are various implementations of this with different GUI’s and whatnot, but they all more or less do the same thing. So far so good for me, mounting with a drive letter on login, then pointing Kopia at the mounted path in “filesystem” mode, but obviously this is just a workaround.
I’m using Duplicacy on the same remote storage over SFTP and it works great, entering the normal fields you outlined in your post.
Let me know if you figure out a way to directly access the storage via Kopia!