Sounds very doable. I’d suppose could, as you say, just use an ftp server. Simplest for everyone is probably just setting up a simple HTTP server with Python:

python3 -m http.server -d /your/rclone/path

Should there be something between RDT-client and Plex? I know that storage isn’t a service/application, but it feels weird that the line goes directly from the download client to Plex

using AD for user management

Do you mean that it authenticates with AD (like with LDAP), or that you want the password manager to manage AD users?