missing/failed connection from Collabora to Nextcloud is definitely a problem - I would focus on this issue. from your curl output connection from Collabora to NC goes to 192.168.1.3:443 which might be the “internal” IP of Nextcloud container - there must be more details related to the cert e.g. subject and SANs included in the cert so you can conclude where it comes from…
as your real public certificate is hosted on NPM you want to connect there (and make it serve the request). I can’t tell you to configure this with TrueNAS but similar topic using plain Docker is discussed here Probably DNS help with NC Docker + Collabora + Wireguard tunnel - you must ensure public domain resolves to the NPM inside of your container so the connection uses proper TLS cert rather direct connection between two containers (or maybe somewhere else)..