> Do you mind explaining why it's so trivial versus setting up traditional ftp?

    nohup /usr/bin/rclone serve ftp --addr MYIP:2121 $PWD &>/dev/null &

No configuration needed (beyond rclone's per-cloud-storage-account config (noting that serving local dirs this way does not require any cloud storage config)) and some variation of that can be added to crontab like:

    @reboot /usr/bin/sleep 30 && ...the above command...

Noting that $PWD can be a cloud drive identifier (part of the rclone config) so it can proxy a remote cloud service this same way. So, for example:

    rclone serve ftp --addr MYIP:2121 mydropbox:

assuming "mydropbox" is the locally-configured name for your rclone dropbox connection, that will serve your whole dropbox.