It's great to hear that the 3 addons worked for you.
It is possible to reverse proxy nextcloud with the letsencrypt addon. Several people have done it and shared their configs on the unraid forums. It is probably easier to use the subdomain method as in nextcloud.custom.url as opposed to custom.url/nextcloud
If you have any issues, feel free to post in this thread: [Support] Linuxserver.io - Letsencrypt (Nginx)