Very low hot water pressure entire house
Your house was built at a time when galvanised pipe was common, the problem is most possibly your pipe has slowly corroded closed in 1 or more places, thus when you turn the hot off the pipe charges open and you loose pressure the longer it is on with a new water heater and old house that is what I would look at a switch over to Plastic with some of the new shark bites is not really that hard and you may get all the pressure you have on your cold by just replacing the main line worth a look and cheaper than another water heator that may end up with the same problem