So the tank is at least 9 years old.
If the T&P has not been replaced, it could have aged. Springs can lose their mightiness.
If you have such low flow that the tub diverter and the sprinklers are giving you grief, then you may need a rebuild or replacement of the PRV.
They can go bad over time.
If your PRV doesn't have a pressure bypass, then you might think about an expansion tank.
I would look first at the T&P though.
Any tank that gives you ten years, had pretty much given it's all.