new heat pump as a replacement for oil heating with existing radiators

Okay, to keep it relatively short: the provider should go with you to the exploded house and show it to you. My guess is that he won’t do that. He wants to get rid of inventory.
Then: at 40 degrees Celsius on the upper floor, your house insulation is not good, but really bad. Or so good that the heat introduced there doesn’t escape. I say the first applies.
Then the question again: why do you want to switch to a heat pump now? At over 80, it will never pay off for you personally.
The annual performance factor is simply a lie, it will get warm, at horrendous electricity costs.
As a "young person" I don’t have to understand your requirements in terms of content. I should listen to them, take them in, and consider them when giving you advice here.
My advice is: keep running the oil heating until the exodus, as long as it works. Alternative: have a used oil heating system installed from someone who is currently renovating.
If you want to do it comprehensively anyway: hit the stop button, inform yourself, inform yourself, inform yourself.
The first step now would be to have a heat meter installed and try how far you can reduce the flow temperature after the thermal balance without losing comfort. Then have radiators selectively replaced so that you reach 40 - max. 45 degrees flow temperature in the core winter. Then in the spring you will have all the necessary data to independently and well-founded get quotes.
If the above is too much for you, I will honestly tell you that a heat pump in your situation brings no savings, you will have a high investment with high ongoing costs, and thus a heat pump is absolutely the wrong choice.

Then: at 40 degrees Celsius on the upper floor, your house insulation is not good, but really bad. Or so good that the heat introduced there does not escape. I say the first is true.

You can insulate as much as you want. After a week of sunshine at 40°C outside, the insulation is "used up". It gets a little better with wood fiber insulation than with mineral wool, but only a little.
In winter, the heat flowing away is supplied through heating surfaces; similarly, in summer, the excess heat must be removed with air conditioning units. No more and no less. Radiators are not suitable for cooling because the room air then condenses on the radiators. You can cool a little with underfloor heating, but you still have the moisture in the air. The air conditioner takes care of that by removing it directly outside.

The exploded house really existed. You are not allowed to link here, but just search for "Frost damage disguised as explosion". I had already mentioned the simple damage prevention.
There is a monoblock air conditioner on the German market, but you have to place the outdoor unit with the R290 inside on the balcony yourself and seal the balcony door—which remains open due to the ventilation ducts—airtight enough so that no heat comes in. Not very practical.
Otherwise, air conditioning manufacturers take the position that R32 is indispensable for them, and therefore they are exempt from the ban. If there is ever a better and equivalent refrigerant, they will use it.

The special application of heating and cooling with the same outdoor unit and no other heat source is indeed not necessarily future-proof. In this respect: yes, he wants to get rid of his stock. I also feel a little uneasy when devices from the Far East are relabeled in "German heat pump factories." The seller is the only German part.

By no means is it the case that well-insulated and non-air-conditioned houses have 40 degrees in the attic at night in summer. I only know that from poorly insulated houses. But the OP can post the actual data regarding the really good insulation, then we will have reliable information.

The house was "really well insulated" in 1990. Since then, expectations for insulation have risen dramatically.

So I built in 1990 and insulated with 14cm Isover (approximately standard) plus wooden cladding under the tiles. However, the upstairs never reached 40 degrees and it was possible to live there normally.

I chose R290 for heating and R32 for cooling. Although the cooling is fully paid for by the subsidy for heating. The price difference per kW is quite remarkable. If one of them ever fails, the other still works until the installer comes by.

Is it possible to do both with one heat pump? What type do you have?