If you want to enjoy hotel type rain showers, get storage heater with pump or solar. Instant water heater won't be able to do that (most Malaysia model) you need high power version which I don't see before.

High water pressure required high power water heating elements.

My sister stay in condo, water pressure is good, she install instant heater with rain shower head, you won't get hot water during rainy day, and warm water during this hot weather.

BTW, some instance water heater will state max working pressure and can't use external pump. This because it won't be powerful enough to heat it.