Why does one of my showers consume all the hot water?
There is a good chance that the lines are oversized for that bathroom.   I dealt with this in a shower in France that I needed to redo.   It was basically close to our equivalent of 3/4&quot; for the cold/hot pipes and I reduced them to a little under 1/2&quot; and it resolved the issue.
If someone turned on both sides high it was out of hot water in 12-15 minutes before we reduced the pipe size, hot water gone from a good size tank.   After it lasted about 35 minutes - which is good for 3 people to shower separately.
The other issue I have ran across is the rain shower heads.   These things seem like water is coming out normal but they are usually so big there is upwards of 3 times the amount of water used.   It boggles my mind how much these things use.
As for your issue, if you remove the shower head and take a picture of the insides it is highly likely you can block off a lot of the flow with a simple rubber gasket/ring.   Without getting into the wall this is your first and easiest option.   A valve from the 60s or even more than 20 years old... I honestly wouldn't touch it as my number 1 piece of advice with intricate hardware - especially old - if it works leave it alone.