I would not change all the piping in the house to 19 mm (3/4 in.). That is too big. What you can do is change the piping that leads to the shower to 19 mm (3/4 in.), and leave the rest as is. Less work will be required and when you are in the shower the larger pipe will give the shower priority. The temperature will continue to change when the other machines are activated, but not nearly as much. Better yet, you could install a pressure balancing shower faucet that automatically adjusts to keep the temperature constant even if the water pressure fluctuates. The volume of water in the shower will change-but the temperature will remain constant. If you do both these things, the larger piping will give you more pressure, and the pressure-balancing valve will protect you from shower shock.