Lowering the supply temperature in underfloor heating more complex than expected?

Confusing menu navigation/operation. Pointless components. Really useful control options are not provided. Apparently a wide variation in quality among the components (possibly an indication of generally below-average quality). Relatively expensive service, which doesn't have much expertise. Too much bureaucracy, the right hand doesn’t know what the left hand is doing. The devices are also too expensive compared to other devices. If the unit breaks down outside the warranty or I can cancel the purchase due to too many repairs within the warranty, I’ll do it like Karsten and get a Panasonic or LG and install it myself.

The approach to reducing the flow temperature with NAT is correct and understandable. However, you should check whether 30° must necessarily be your goal. Maybe 32° would also suffice. When designing underfloor heating, the standard room temperature of 20° and bathroom 24° is often used. Here as well, you should reconsider the 24°. 23 or 22° make a big difference for this calculation. Then you have to consider the location of this room within the entire house. Two external walls in the bathroom have a different effect in the heat load calculation than only one external wall and three internal walls to rooms with 20 or 21°. Due to your desire to reduce the flow temperature, the pipe spacing will be smaller. If you choose 10 cm everywhere and 5 cm in the bathroom, then you are close to a very low flow temperature. From my point of view, it will remain at the Arotherm Plus 55/6 or even 35/6. In any case, it is good if you closely monitor the design of the underfloor heating and the selection of the heat pump type. Then later you will be less likely to vent your frustration at the manufacturer.

Relatively expensive service, but they don't have that much knowledge.

Doesn't the service fall under warranty services? In which cases do you have to pay for the service?
Regarding the control, there should be some adjustment options via Modbus or EEBUS. What would you like to control on the heat pump that is currently not possible?

You should reconsider this 24° here as well. 23 or 22° make a big difference for this calculation. Then you have to consider the location of this room within the entire house. Two exterior walls in the bathroom have a different effect in the heat load calculation than just one exterior wall and three interior walls to rooms with 20 or 21°. Due to your desire to reduce the flow temperature, there will be smaller pipe spacing. If you choose 10 cm everywhere and 5 cm in the bathroom, then you are close to a very low flow temperature. From my point of view, it will remain with the Arotherm Plus 55/6 or even 35/6.
In any case, it is good if you closely accompany the topic of underfloor heating design and heat pump type selection. Then you will have less reason to vent your frustration on the manufacturer later.

What makes us suspicious is that the building description says that it is generally designed for 35°. Our bathroom is on 2 exterior walls. We will definitely address whether lowering the room temperature in the bathroom would be a way to reduce the flow temperature. In any case, I already take with me that even 35° flow temperature would not be the end of the world :).

If possible, have the (room-by-room) heating load calculation for your property provided. Then you can consider options. Above all, you can check yourself why the builder wants a five-figure amount as an additional charge ;). In your bathroom and 2 AW, it may be that with a design temperature of 32° you can manage with VA 5 cm (please also install under the bathtub and in the shower area underfloor heating). A wall heating can also help. The proper design of the underfloor heating is the be-all and end-all.

Is the service not covered under warranty? In which cases do you have to pay for the service?
As for the control, there are supposed to be some configuration options via Modbus or EEBUS. What would you like to control about the heat pump that currently isn’t possible?

You get "service" through a maintenance contract.
The thing with the energy integral is more of a pain.
It just cycles a lot.

addressing whether reducing the room temperature in the bathroom would be a way to lower the flow temperature.

In my opinion, a very bad idea.
That’s just a sleight of hand that doesn’t make the house or bathroom any degrees warmer.
It also doesn’t improve efficiency.

(please also install under the bathtub and in the shower floor heating).

If the tub is in a styrofoam box, floor heating underneath is rather counterproductive, since the loops reduce the flow without adding heat.

A wall heating can also help.

That helps a lot.

The warranty doesn't last forever. And after that, you just pay at least 400 EUR for a replacement.

In my case, it was the commissioning without consultation with the installer (actually debatable, but since I still needed a document from him for the BAFA, I didn't complain about 450 EUR considering the over 13,000 EUR in subsidies). I killed the safety temperature limiter because the hot water valve was closed, but I had turned on hot water production. The heat had nowhere to go and brzzffft...
Only the technician had actually told me that I could turn on hot water anytime. And there are two valves. He showed me one, not the other...

So I'm a bit behind on this because I have an incorrect calculation of yield and efficiency numbers and therefore haven't properly gotten into it yet. But what the colleagues in the neighboring thread do with many pages and workarounds, I imagine there should actually be more direct possibilities.

And via the ebus, you can, for example, configure such simple things as: when there is a photovoltaic surplus, heat my hot water storage really strongly and also heat my screed a bit, only it's very complicated to set up. Actually, you need programming knowledge for this, real scripting knowledge. Not just clicking together some Alexa routines.