Yes, as most users would, me included, you have just used the precompiled firmware and just loaded it in when you updated to the latest SDK. And that’s the way 98.75% of users will do this too. And yes, that number is just made up.
There are a couple of good threads on building the Cerb firmware, this one is a good one to start as it has pointers to the community maintained source tree:
But this one https://www.ghielectronics.com/community/forum/topic?id=12488&page=1 and https://www.ghielectronics.com/community/forum/topic?id=10015&page=1 are also totally relevant to compiling it yourself as well - a little light reading to see if you think even compiling this is a task you’d want to take on. Then, you really are likely to have to dig into the porting kit, and you’re going to have to uncover how the current serial ports are mapped from hardware to software, and replicate/extend that to add the additional ports.
the problem with just looking at processors with more peripherals like the Hydra’s, is that you may still need to delve into the porting kit unless you can find one that is actually already supported (has an active port in the porting kit that you can use, plus it exposes the additional serial ports you need). I honestly think the best approach would be the community ports for the Cerb family, as there’s a number of people now who are active in that effort and may even have the knowledge or have done the porting kit plumbing already. Certainly the design files of the Hydra are available but it’s not as simple as the Cerb processor to re-purpose.
One more potential option (talking without knowledge here, stand clear this could be dangerous) is the processor on the DL40 is a reasonably powerful device, and I suspect it could be re-tasked to do the 3rd UART job and pass you back the data. Taylorza is the guru DL40 guy but has recently moved country and bought home a new baby to his household, so hasn’t been too active here of late, but he may see this… the DL40 thread is https://www.ghielectronics.com/community/forum/topic?id=10325