Big rain shower with instant hot water, you will face 2 possible problems:
1.Water pressure not strong enough, so the flow of the rain shower will be slow.
2.Water pressure strong (maybe you have household water pump), but the instant hot water can't heat up the large volume of water.

rain shower does not need high pressure water.

if water pressure is enough for normal shower head type, it will be enough for rain shower system.

Your instant heater pump cannot coupe with this big shower head. It not enough power to pump the large amount of water for this big shower head.

The most bigger size for instant heater is 7" head. The istant heater have to design for the shower head, Not any instant heater.

Another problem is the water temperature will drop alot. This is due to

1) high flow water cannot absorb enough heat from the heater.
2) Because of the slow water drop from shower heat, the water been cold down by the air between the water travel from the head to your body. You can feel the water colder than using the hand shower.

Use water heater tank.