Maybe not compared to a power shower but excellent compared to a bath shower mixer tap which many people still use for washing and rinsing hair. So I would say an electric shower of the right current is absolutely fine.4 litres per minute isn't good.
Doesn't have to be done all at once.
Use the combi for a mixer shower today
add a hot water cylinder or some other HW storage device later (can be heated by the combi)
ASHP when the combi needs to be replaced.
Blup