New shower lukewarm - balancing issue?

[dilalio]

Or do you always eventually need a hot tank if you want lots of hot water flow?

Yep.
And even that runs out eventually, depending how long, how much and how hot!

 

[dilalio]

I turned the cold isolator that feeds the shower, just a bit, to reduce the pressure.

That reduces flow, not pressure!

 

[dilalio]

Oh.... Quantity of hot water with Δt 30°C = 13.2l/min. But, but... I had 2 showers in my old house and didn't have this problem.

It's physics pure and simple.
If you were to restrict the incoming cold main to the boiler to 13.2lpm, measure the temp of the water at a cold tap, then temp from a hot tap, it should be 30°C higher!

So, for example, 10°C cold main = 40°C hot water only if flow rate is 13.2lpm.

Increase flow rate = decrease temp rise (Δt)

Boiler has a problem if this is not what gets delivered under those conditions.

Your old house likely had better temp rise or lower incoming flow rate (if it was a combi).