Defensive offer, or have house prices become so expensive?

In today's market situation, everything except bidding procedures is simply money left on the table. No one can value it seriously anymore.

We sold our last house six months ago. Without a bidding process, because we didn’t want it. Since we have bought and sold several times ourselves, we found that off-putting. It’s not everyone’s thing.
So we thought of a price that would make us happy if we achieved it and also yield us a small profit – which is normal and almost "expected" at this point in time, as long as the house is not ready for demolition.
We did not set an exaggerated price and therefore immediately had some interested buyers who wanted the house and also had the appropriate financing. That was important to us; we now live at the other end of Germany and didn’t have time for many nerve-wracking appointments. We preferred to sell quickly and securely. Although, of course, we could have gotten significantly more if we had had the nerves and patience for it. But we didn’t want that for ourselves. And no “haggling” either – precisely because we ourselves can’t stand when realtors or owners do that.
I assess you similarly. So take a price that gives you a little “feeling of happiness” when you achieve it, but that is not excessive in your eyes. You can see what’s on the market for you. 450k for a 30-year-old house – personally, I’d easily try 550k if the location and plot are correspondingly good/large. I have some pictures in my mind of your house; it really looks nice and certainly appeals to many.
Oh yes – and of course, it would be an advantage if you already know what you need to get at least to be able to build or buy your new home with a clear conscience.

In today's market situation, everything except the bidding process is simply money left lying around.

No, money left lying around is money left lying around, the bidding process is the bidding process. You don't have to link the two.

No one can appraise that seriously anymore nowadays.

Yes, it would gladly be easier than it currently is. But there used to be more frills, and everything passes. If it were easy, everyone could do it ;-)

No, money left lying around is money left lying around, auction procedure is auction procedure. You don’t have to link the two.

Yes, it could definitely be easier than it currently is. But there used to be more frills, and everything passes. If it were easy, everyone could do it ;-)

You don’t start at €1 eBay-style ;-)
The minimum bid is the 500k that Kati would feel comfortable with. The rest is then decided by the market.

You don't start eBay-style at 1€ ;-) The minimum bid is the 500k that Kati would feel comfortable with. The rest is then regulated by the market.

I know someone (but he doesn't listen to me, and even less to his wife), who would quite like it if it were eBay-style: then he would have practically already "sold" his condominium with "30% compensation for fun bids" after three wannabes ;-)

A house, gable roof, solid construction with about 150m², no balcony, one side with a kind of "Frisian gable", KFW40 standard with air-to-water heat pump and central ventilation, was priced for us at around €400,000 - pure house price, turnkey.
Neither earthworks (so nothing at all!), nor floor coverings / painting works, no photovoltaics, no air conditioning, no frills are included. Otherwise, the construction service description seems quite solid.

Welcome to the South ;)
I have about 410k, including earthworks, flooring, and painting, but a bit smaller (133sqm living space + technical room) and completed a year ago. Since then, prices have risen sharply. And not only that... some construction companies are still stuck with fixed prices they agreed on beforehand and now have to complete unprofitable jobs. They make up for it elsewhere.
And take a look at the prefab builders and check out the promotional houses. Compared to our search, they have already increased by 20-25%. And that’s without extras yet.

You shouldn’t overestimate cheap area -> cheap area either. Craftsmen are mobile, and construction prices adjust faster than land prices accordingly. My general contractor has also already built a house in a suburb of Konstanz, which is almost 50km away from them. Almost all country roads. Only when you have a large inexpensive area does it eventually become significantly cheaper. And construction costs rise independently of land prices. I’m surprised you can still get a plot so easily anyway.

Or is it the jump from KFW55 to 40 that costs so much money?

Depends on the house and the construction. In our case, only a bit of insulation under the base slab was missing, the construction company then booked that under customer acquisition. Styrofoam is cheap and the work is the same.
But if a house just barely meets KfW55, then of course the jump is more expensive.