Now the big issue is damaging the water proofing Usually a rain shower is used by itself so not an issue .

Are you doing a complete gut remodel? Or, do you have access to the other side of the wall to do all new plumbing for everything? Are you adding another water heater to take care of the large hot water demand? Adding a separate pressure tank? Upsizing the drain?

Do you have water restrictions about sending 7.5 gallons of water down the drain a minute? Most places are following CA’s lead and require only 1 low flow fixture in use at a time. Because its ecologically irresponsible to send 7.5 gallons down the drain a minute, even if you can afford the water and energy bill to do so.

There is access to the other side of the wall to get to the plumbing. We're re-doing the closet as well so cutting in the wall isn't a big deal. I was not planning on adding another water heater, pressure tank or enlarging the drain.. our pressure is low in the house/area. Would a pressure tank help with that?

Damaging the water proofing?

Like others have said, adding a shower head will exacerbate the heat/pressure issues you are describing. The only way to fix is to increase your supply, which might involve repiping a large part of your home.