I don't know about FreeNAS jails but transferring your existing Nextcloud config to docker might require some tinkering.
Usually Nextcloud on docker is made up by two containers:
- The DB, usually MariaDB
- The Nextcloud front end.
You can actually import MySQL dumps into a container, so I wish you good luck in attempting this😀
Regarding the SSL paranoia, use thelet'sencrypt + nginx image made by linuxserver It is great: will manage your SSL renewals, out of the box has a very good SSL config (not perfect but close) and most importantly comes with a huge list of presets to reverse proxy popular applications, including Nextcloud which is itself a beast.