Water flow in shower is low

Dear forum,

since we moved into our house, which was already the case in 2016, it often annoys me that the flow rate in our showers is very low. For your information, the bathroom is located on the 1st floor and a Grohe Euphoria 180 is installed. Now the pressure when showering is quite low, which is not too bad when using the "rainfall" function, but when using the handheld shower, it also feels like little water comes out of the pipe. Our home builder says that it is due to the water-saving functions of the shower. Now I have measured the flow rate at various points in the house and wonder whether it really is because of that, or if the pipes were simply undersized?

I recorded the following values (100% cold water):
Kitchen: 6.5 L/min.
Bathroom bathtub faucet: 10.5 L/min.
Bathroom shower handheld: 8.5 L/min.
Bathroom sink (2 faucets on one connection): 8.5 L/min.
Garage 3/4": 21 L/min.

According to the pressure gauge after the backflow preventer, the pressure is 3.5 bar. An increase is not possible. It should be noted that the domestic water passes through an ion exchanger, which according to the datasheet should deliver 8 m³/h. However, only 1/2" steel flexible pipes are connected to it. Since there is by the way a mixing valve installed, I can close off the ion exchanger, the flow rate is then as follows:

Bathroom bathtub faucet: 14 L/min.
Bathroom shower handheld: 9 L/min.
Bathroom sink: 10 L/min.

That means that the ion exchanger at least somewhat restricts the flow for cold water. Does it therefore make sense to change to 3/4" or even 1" steel flexible pipes there? Now, for example, a "normal" faucet in the kitchen generally only has 3/8" pipes connected, are these values plausible? Is it also possible to go with a larger diameter there?

Is there something else that remains to be tested, and what options do I have to increase the pressure in the shower? Could it perhaps also be that the pressure feels lower because the water is simply "softer"? The water hardness is set to total hardness 4° dH.

I am very grateful for any tips.
Best regards

Before I enlarge all the pipes, I would, solely for cost reasons, look for a rain shower that provides a better sense of volume with less water. There are definitely differences.

Try whether it makes a difference if you unscrew the hand shower from the hose and measure directly from the hose. Many shower heads and also hand showers have an Eco version (also from Grohe). If this makes no difference, you can already rule out the shower fittings.

Run the water with the main shower and also at shower temperature, not in cold operation.

If you unscrew the showerhead, there may be a reducer between the showerhead and the hose.

Addendum: the aerators are usually water-saving aerators. Run the washbasin without the aerator.