Terminology: In the HVAC world "furnace" refers to heat delivered by hot air (ducted or ductless). What you have is always refert to as a "boiler", not furnace- the terms are not interchangable.
Low water heating performance from embedded tankless coils in boilers tends to be closer to the rule than the exception. It may very well be that the new aquastat was set lower than what it takes to get adequate hot water performance, whereas the hot water performance was adequate back when the boiler temp was ...getting too hot...".
Before boosting the boiler temp, see if you can't fix it at the shower end. The fact that the problem seems to be limited to just one shower points to the shower mixer (which is
not the diverter). Most mixers have an adjustment screw to limit the maximum amount of hot water that can be mixed in. The Symmons mixers aren't all identical but they're pretty similar, see if you can't just get there by
adjusting the hot water limit screw. If that doesn't do it, try raising the water temperature.
MA code for new installations requires a thermostatic mixing valve or tempering valve on the output of the water heater (any type), and the installer is required to set it to something like 115F (I'd have to look it up again to get the exact temp) for distribution to sinks and bathing taps, but dishwashers and laundry is allowed untempered water. If your tankless coil has a tempering valve on the output, turning up the temperature there is the first step.
If that fails or you don't have a you may have to bump the boiler's temp to see if it makes the difference. Raising the temperature of the aquastat (particularly the low-limit) will improve hot water performance, but before you just crank on it, what are the current aquastat settings?
There may also be an issue with the anti-scald valve (if you have one.) The Temptrol series has an internal anti-scald valve. If that is sticking or if there is a lot more pressure on the cold side than the hot, that may be limiting the hot water flow too.