Using the basci 70% rule, you get 35 gallons of water before the temp available drops to a level not warm enough for a shower. The most obvious answer is shorter showers and change to a 1.6 GPM shower head.
The next thing is to make the water heater HOTTER......BUT with a big caveat! It is just plain dangerous to send water hotter than about 130 out of the heater. What you must do is have a plumber install a tempering valve at the outlet of the heater. This will cost a lot less than a new tank or a tankless, and will stretch the available water.
Sounds like you might not have room for a second tank in parallel. So the final option is a tankless. Also the most expensive, by a lot $$$$$. Make sure you get a tankless sized for your demand. How many showers might be in use at the same time , along with a dishwasher, etc???