I just wanted to follow-up, and not leave you guys hanging.
I realized that only Web Traffic needed to be behind the Reverse proxy (for the WebIF), whereas SIP and RTP did not. I am already using split DNS, but I setup one DNS entry for PBX.fqdn that points to my reverse proxy, and SIP.fqdn to point to my actual server. That way, my phones can be directed to the sip server, and my web browser to my proxy. Done.
However, since I disabled all IPv6 traffic on my network, I was having issues connecting from outside, as was mentioned.
Now, I have the PBX system moved to a $5/month cloud server. Time will tell if it has enough resources to accommodate my usage. It has a setup similar to the aforementioned.