I've done some maintenance this week to clean things up a bit and get things back in order. I've gotten my phosphates down to ~ 300 ppb now and will get them down further. pH is on the lower end and I'm going to hover there (7.2) for a bit as I found scale deposits at the bottom of my pool (the robot picked some up this morning). I took all my pipework apart, flushed it and checked for obstructions. I don't know if the scale I found (assuming it is) is from the heater currently or was sitting somewhere in the pipework and I simply flushed it out. I think the cleaning of the system has helped a bit. The heater no longer whines when it fires, however the SFS temp still rises, just not as fast. It starts in the low 300s then gradually rises. If I go out and check after an hour, it's around 360 then slows down further from there, but it will reach 400 after a few hours. I realize flushing things out likely didn't affect this, but I also properly bled the lines to ensure there's no air pockets in the system. The water temperature does rise at the rate i'm used to (1 degree F per hour, which is roughly the rate it should be, or close to).
As for the gas pressure, the guys that installed my line are coming out for an AC tune up and can check the pressure at the heater as well and hopefully adjust the mix if needed. It's not something I feel comfortable doing and shouldn't be anyhow. If something is occurring at this point, it's likely sooting. I did notice ash flakes on top of the heater cover from running it earlier in the week, when I first came back here to report more issues. That supports a burn/pressure issue I assume. I haven't seen this since then. I hadn't run the heater since my chemicals were a bit off but did fire it up this morning.
Now, as far as sooting - is the only way to clean that a disassembly of the unit and brushing it out? That's a good amount of teardown and I'm not sure what the warranty allows me to do, plus I'm not very comfortable with that based on videos i've seen and watching one of the heater techs do it - even he forgot to connect a ground wire when he re-assembled it.
Thanks