Wow, that’s amazing work. Sounds like you’ve almost completed making Matrix-Synapse a fully functional one-click deployment. That’s awesome. I look forward to getting the benefit of your work in future releases 
I was totally defeated by Coturn so I went off exploring in other areas - I found that eJabberd has a STUN/TURN Server that is not turned in the default Freedombox config, see below!
Matrix-Synapse vs XMPP
I had originally started exploring Matrix-Synapse because I’d been having trouble navigating the XMPP world - so many clients and no standard set of features. For example there is no suite of compatible XMPP clients for Android, iOS and Linux that support both OMEMO encryption and Jingle voice/video calls. Yes, you can install Jitsi client on Linux and Android and make voice calls, but there is no compatible iOS client etc etc. I need something that deploy-able cross platform from apps - not just in theory, but in practice. Matrix-Synapse seems to provide this (The most comprehensive list, comparing of XMPP client features, I found is here: https://riseup.net/en/chat/clients).
eJabberd STUN/TURN Server
I found that eJabberd has a buil in STUN/TURN server that is pretty well documented here: https://docs.ejabberd.im/admin/configuration/listen/#ejabberd-stun-1 but it is turned off in the default Freedombox eJabberd config file: sudo su, nano /etc/ejabberd/ejabberd.yml I was going to start exploring configuring it to support XMPP Jingle voice and video calls.
OpenFire
I also tried OpenFire XMPP server. Super easy to deploy (even for a noob like me) and nice web admin GUI (also great for noobs). It has a built in media relay and easy to install plug-ins for Jingle and TLS/SSL certificate management: https://www.igniterealtime.org/projects/openfire/
OpenFire posted an update yesterday (26/4/2020) providing guidelines for implementing Coturn for OpenFire, in preparation for the XMPP Android app, Conversations, releasing an update next week that will support voice and video calls. See blog post here: https://www.igniterealtime.org/
Conclusion
I’m eventually going to do a blog post about what I’ve discovered about the state of XMPP, but in the meantime, it sounds like you have made voice and video chat a whole load more accessible to none techies via Matrix-Synapse. That’s a massive thumbs up for Freedombox.
Thank you