Hello, I just wanted to add to this topic, since I was looking for the same info, and found another possible solution.
Instead of trying to edit the config.xml with a regex/sed, it seems simpler to use the approach featured in this github repo. Use a php script and the built in functions for editing the config.
Check out
https://github.com/zxsecurity/pfsense-import-certificate
You will need to install the script on each firewall, and then upload your certs, and then call the script. For centralized letsencrypt managment this seems like it could be a good approach. I have 30 firewalls and I don't really want each one running acme, I would rather run a central letsencrypt, and deploy the certs to each firewall.