I used the one in my signature for inspiration -
https://forums.freenas.org/index.ph...-to-reverse-proxy-your-jails-w-certbot.49876/ - although mine is configured a little differently.
I'm using subdomains (e.g. nextcloud.domain.co.uk) rather than subdirectories (e.g.
www.domain.co.uk/nextcloud) and have Letsencrypt certificates that auto-renew for each subdomain. I think Letsencrypt do wildcard certs now, but mine is working well so I haven't investigated.
I also have nginx.conf files for each service using sites-available/enabled as I find this easier to maintain, although it's probably more work to set everything up rather than having 1 big nginx.conf file.
That guide should at least get you heading in the right direction. Setting up NGINX as a reverse proxy with Letsencrypt renewals in a single jail was one of the best decisions I've made with my FreeNAS box. It makes running multiple services through port 80 and 443 simple, and once it's all set-up it just runs away in the background.
Good luck...