Hey @rjarratt welcome to the Nextcloud community and thanks for supporting Nextcloud snap 
you’re obviously using the snap 
that’s great
you’d need to pass through to be able to let the snap manage SSL and thats not so easy!
for your use case a reverse proxy managing certificates would be the easiest, see
Reverse proxy configuration and Hosts & FQDN configuration recommended NGINX proxy manager reverse proxy with termination
not necessarily, as forwarding Nextcloud snap through the reverse proxy will be using HTTPS too and nothing will change?
Below are some links to how-to’s answering frequently asked questions and may help you help yourself;