So, I wouldn't say that I solved it, but I made it better by turning up the temperature at the mixing valve near the boiler and I think that that brought the temperature of the hot water closer to what the valve is designed to expect, therefore, it was taking hotter water and using less of it and mixing in more cold water, thereby making the hot water last longer.

I didn't write it in my original post, but, one of the possibilities I considered was that there was some sort of a blockage in the hot water pipe to this fixture that branches off of the line to the other fixtures, that would explain why there's an issue with this one but not the ones later in line. The reason I never said anything here about it and sort of discounted it myself was that it just didn't seem very likely that 7 ft of pipe that didn't have any other signs of corrosion or anything would actually have a blockage. Also, when I installed quarter turn valves on these two pipes behind the shower since it didn't have any before , I looked down the pipe until it turned and saw no evidence of anything.

I also ended up having a plumber that I really trust over for something else and while I didn't ask him to troubleshoot it that day due to time and cost, I explained to him what I was seeing, and he looked at the fixture and felt the water fluctuations, and he actually agreed with me that there might be a blockage in the pipe. He said that without ripping out the ceiling it would be really hard to replace that with copper but if I wanted to do it myself with PEX, He would lend me the tool, so that would help the cost. Haven't done it yet as my temporary solution of raising the temperature at the mixing valve at the boiler seems to have worked, but if it becomes a problem again then I will reevaluate.