I use KasmVNC with some container apps that bundle it, but that doesn’t really match RDP’s feature set.

Have you setup the Kasm Server aspect[0]. That is, not just the container workspace, but you can have Kasm connect to a separate actual server via RDP, or VNC; not a docker container workspace. In this manner, Kasm has completely replaced Guacamole for me.

[0] https://kasmweb.com/docs/latest/guide/workspaces.html#server...