Upstairs shower running out of hot water almost immediately
You just replaced the tank so it shouldn't be a broken dip tube as I had first assumed. My next thought is there might be a blockage somewhere, but you should notice that as a loss of total pressure (compare the flow rates of your hot and cold coming out of a faucet somewhere). 

You should probably remove your shower valve's cartridges and clean them while you're purging the entire system and flushing out debris.

I have a single handle shower valve that doesn't get as hot, or for as long as I'd wish, so I have cut back the cold water by closing its supply valve a little bit. This adjustment alone has increased my acceptable temperature dilation from ten minutes to upwards of half an hour.