Folks here might find this useful, either as-is or as an API reference:
https://github.com/0xabu/opnsense-cert
(https://github.com/0xabu/opnsense-cert)
It's a moderately-simple Python script that automates import of a new TLS certificate and supporting CAs, then restarts the webgui service, all using the API. (In the context where I use it, another machine on the local LAN manages renewal and installation of these certificates.)
Thank you for posting this!
I've been working on the automation of my certificate renewals and this showed up at a good time. Your script is well thought out and now I am using it for the opnsense renewals.