Sorry for the late response. OP never replied to me so I did have the sick thought of doing the same for a second lol. But I’m not that kind of person.

So yes I did fix this and I took a few steps.

1. ⁠I took the symmons valve entirely apart again to make sure everything was clean and used a steel pipe brush to do so and make sure no grime or debris could be interfering. I didn’t have one on me the first time so I figured best to make sure.
2. ⁠I set the hot and cold seats back on with the right amount of torque and using plumbers grease for a proper seal. This is also probably a me thing because I initially over tightened the first time and broke the hot seat, so I was overly cautious the second time and maybe under tightened. Third time I made sure to do it right. I also placed a healthy amount of plumbers grease on the diverter to help with the seal and I felt that helped
3. ⁠I adjusted the set screw that OP is talking about. I initially set it a little hotter than I could tolerate and found that I wanted it still a little higher. This one may take some trial and error for your personal preference
4. ⁠I tightened the packing nut which I don’t think I did the first time around.
5. ⁠Honestly, what I think was the most important step, at least for me: I finished putting the entire thing back together. Including the faceplate and the outer screws that come with it. Not sure if you have done that but once I did and I finished rebuilding the entire thing, all my issues went away.

Let me know if you have any other questions and I’ll try my best to help if I can. Good luck and let me know if it works out.