That's a good sized heater. It could be filled with sediment and not working efficiently. If you haven't emptied it recently, it might be worth it.
If it's clean, you can try turning it up. It will make full-hot from your faucet hotter, so be careful of scalding yourself, but it will make the hot water last longer.
You could also get a slightly lower flow shower head. Using less water at a time will allow it to last longer and keep do better keeping up with the empty rate.
If none of that works or is something you want to try, you could always replace with a tankless and never run out. Though you'll waste a bit more water each time you turn on the hot since it takes a few seconds to kick in, but you won't run out.