A couple of things. Pressue will be flakey until you can get rid of the galvanized.
All new shower valves have anti-scald technology to help prevent that. They adjust the balance between hot and cold as the pressure drops on one side to keep the temperature fairly even, so replace the valves. At a minimum, it will have a pressure balance valve built-into it.
Personally, I prefer an enhancement on that, a themostatically controlled valve. It does about the same thing, but you can set the desired temp, and winter to summer, it stays the same.
To extend the amount of hot water available, you'd need to do two things: turn the thermostat up AND install a tempering valve on the outlet of the WH. That is an adjustable valve to mix some cold into the hot to 'temper' it to a safe value. They generally come from the factory set at 120-degrees. But, if you are going to replace the WH, you should consider sizing it to the expected use. It's still not a bad idea to run the WH hotter, since at 140 or above, typical bacteria and virus can't live (there are exceptions, but they're rare).