you can add a pump. That will help alot but cost is high. but yes the plumber is right a bigger tank helps the pressure because the weight of the water up above. your old house may have had a different hot water system and better pressure. You can make the tank higher but how high can you go...
I am taking that they don't have a hot water cylinder in the house? check what the pressure bar is on the boiler. does the pressure build up fast again after running the tap.
Try watering down the colour paint you want to put on the wall. if its not trade paint only water down a little if its trade paint a 1/4 of a cup will do and mix well. should be ok if not yes rub down wall .