Construction costs are currently skyrocketing

I always find it strange when the Merit Order is criticized (rightly) with the argument that it is not a free market (wrong). The Merit Order principle simulates the free market in the case of excess demand. You just have to imagine it with apples. A farmer can produce 10 apples a day on his plantation at a price of 1 EUR (gross retail price) each. In the village, every person wants to eat one apple a day and there are only 10 inhabitants. It works out. Now the pear farmer comes along and also grows apples but can only produce apples on his pear-optimized plantations at 2 EUR each. As long as the demand is only 10 apples, he naturally won’t sell them (unless they taste better, but let's leave that aside). But once a year there is the apple cider festival and during the 10 days people need 2 apples per day, one to eat and one to drink. What happens then? Does the apple farmer sell his apples for 1 EUR and the pear farmer his for 2 EUR? No. Both can achieve a higher price until the demand is met. The pear farmer earns more than he otherwise would, since he now sells apples in addition. The apple farmer also earns more, since he makes more per apple.

Electricity is not a storable commodity and only poorly storable, but above all you don’t want supply to exceed demand (grid stability), so actually no one should produce more than they can sell. But if the demand (electricity demand) is higher than the supply of the "cheap providers," this should also be produced and at the same time the cheap providers should benefit from being able to produce more cheaply, as in a free market. That’s why the Merit Order principle exists.

I’m not saying that this is good and right, but Merit Order initially has not so much to do with market distortion, but with market simulation.

Does the apple farmer sell his apples for 1 EUR and the pear farmer his for 2 EUR? No. Both can achieve a higher price until demand is met.

We are currently more in the area of, oh, the pear farmer ran out of fertilizer and raises the price to 10 euros. Besides, we don't need 20 apples but rather 11-12, and the 1-2 extra apples cause the others to also be priced at 10 euros... So in other sectors without a merit order, prices would generally rise sharply in case of doubt, but you also don't want to scare off the customers, so maybe not quite that extreme, not that fast. In the long run, this usually causes either demand to drop ("everything is just too expensive! I won't eat apples anymore!") or supply to increase ("look how much the apple farmer is earning right now! You can make a ton of money!"). Presumably, this also happens with electricity sooner or later, only it doesn't all happen that quickly and maybe the new apples don't taste that good... uh... okay, maybe that was taken a bit too far :D

Yes, that's exactly why I also don't say that it is so good. But it is not a totally unfair market because of the Merit Order. Instead, the market is unfair.

And yet, if the demand is that high, then the last two apples would determine the price of all. Assuming transparency.

He refers to the past, when after the 10-year fixed term of his loans, he always had an advantage by simply "letting it run."

Well... in principle he's right, when it comes to the comparison with external banks. They, meaning the currently financing bank, of course make you an offer roughly 0.2-0.5% cheaper than another bank. That is a service advantage because they know you and the property. New banks have to check you and inspect the house... those are personnel costs.
However, the offer will always be adjusted to the time when the contract is to be concluded.
So if the average in 2024 is 6%, then your bank offers you 5.8 or so.
If you now conclude 2 years forward, then at today's interest rates plus 0.25% per year surcharge for the forward.
I cannot imagine that anything will change to your advantage in the medium term. However, I am a layman here, others are better informed.

Last week, Madame Lagarde announced further hikes of 50 points. In this respect, interest rates will continue to rise.

In the end, it is important that one can afford the future repayments. Striving to minimize losses by crystal ball gazing is, in my opinion, not advisable for long-term investments. Why not diversify and cover part through forwards and maybe a smaller part in the MSCI World via ETF and possibly special repayments in a proper fixed deposit? After all, you do not usually invest your funds entirely in ONE asset class. So why here?