ᐅ Bathtub Despite Limited Water Heater Capacity?

Hello, we have purchased a house and want to renovate the bathroom. We would like to install a bathtub. It should be a corner bathtub with dimensions of 1.40m x 1.40m (4 ft 7 in x 4 ft 7 in) and a water capacity of 290 liters (77 gallons). However, our heating system is located in the attic and has a wall-mounted storage water heater from Vaillant (vih cb 70) with a 63-liter (17 gallons) hot water reserve. This will definitely be too small. What options do we have now? A smaller bathtub?

I would appreciate any advice or suggestions!

Cold bathing would also be possible.

A completely new concept for hot water is clearly needed.

An electric tankless water heater would be my first thought.

We once had a 70-liter (18.5-gallon) hot water tank, which was just enough to heat a small standard bathtub (about 200 liters (53 gallons)). If someone had washed dishes in the kitchen beforehand, you would end up sitting in a puddle, and then the water would already be cold...

During the bathroom renovation, we chose the Kaldewei Studio Star bathtub for space reasons. It is taller than others, oval-shaped, and very economical with about 150 liters (40 gallons). It was expensive, but we could enjoy a hot bath in it.
Therefore, I do not recommend extra-large soaking tubs where the water cools down before you even get in.

Simply just a different heating system / larger storage tank.