Inconsistent hot water

Hi all,

I’ve been having issues with my water heater lately. 50 gal Rheem gas unit, ~10 years old. At times, I get nothing but lukewarm water from it, particularly in both of my bathrooms. But it’s not consistently lukewarm. For instance, my wife got home this evening and took a bath. Filled the tub and it wasn’t hot. Then I started doing the dishes right after and the water in the sink was totally fine and to temp.

I’ve never flushed the unit and when I asked my plumber about doing so, he said it’s not common for our area. I’m on city water. Maybe a cracked dip tube? But then wouldn’t it be consistently not hot? Supply lines are 1/2” copper which I know is undersized and I’m in talks with my plumber to upgrade everything to 3/4”. Farthest fixture from the water heater is maybe 35’ away.

Just kinda stumped. Thanks for any suggestions