Nah, no hidden special branch for paying customers, it's mainly just documentation and more of a direct line to the FusionPBX team (although most of them read this forum anyway). There's a few member only apps, but I don't find them to be game changers myself.
Cross my fingers and hope on a master that I keep a little bit locked in time with some custom work to it. I can always branch in a few fixes too if necessary.
Personally, I think the whole project would need a bit of an overhaul or a redesign architecturally for seamless updates, probably with containers. It's not so much FusionPBX has dodgy updates, but more that all the moving pieces have ABI/API breaking changes. Everything needs a 'stable' here, and nothing really does in Debian land.
Take the recent update to FreeSwitch where SignalWire added CPU usage into the log file, well this breaks Fail2Ban, and now your intrusion protection is busted. You update PHP, and snuffleupagus might go and break previously working code, you update nginx and WebRTC magically breaks from some sort of TLS thing, etc.