Contracts signed - doubts within the withdrawal period

Nonsense. That may be an idealistic aspiration, but it has absolutely nothing to do with the current market situation.

And yet there is more truth in house construction than you are trying to talk me out of right now.

Please. Not everything that limps is a comparison. Fully developed modular cross-construction kits, brand image, target group are all factors that affect the price more than the performance/quality.

Is that now positive for Dacia or for Audi?

Of course there is a difference in price, quality, and workmanship. If you really want to sell me something else now, I won’t take you seriously anymore.

And yet there is more truth in house building than you are trying to talk me out of right now.

Again. No. The market is currently enormously driven by supply and demand. A craftsman or BU with full order books inevitably charges different prices than someone who urgently needs orders right now.

Is that positive for Dacia or for Audi now?

Positive for whoever can utilize synergy effects. For Audi, that's its developed MBQ; for Dacia, the fact that a Renault Clio is inside the Duster. If Audi, for example, had to completely redevelop all system components of an A3 from scratch, the finished product would be significantly more expensive than an MBQ-A3. At the same quality/performance. Saying volume, cross-subsidization, ROI.
The overhead also has to be controlled. Audi is a premium partner of FC Bayern; for Dacia, Scholl antics on the Sportschau commercial break over the screen. The different marketing costs that must be covered again do not directly flow into a quality or performance difference.

Of course there is a difference in price, quality, and workmanship.

To stick to the (really bad) example of the automotive industry:
VW Up!, Skoda Citigo, Seat Mii. - All VW120 -

Three times the same car with minimal design differences, all of which come from the same factory, which can switch models within a shift. The manufacturing costs for the different manufacturers probably differ by only a single-digit euro amount at most, yet the car with the VW label is between €1000 and €2000 (depending on the equipment version) more expensive than the Seat, i.e. 10 - 20%. Differences here are found maximally in brand image and target group. Quality, workmanship, material... SameSame

b) ["Radical solution"] Withdraw from the contract and go to my 2nd favorite (another developer, almost identical price),

Clearly "d". Withdrawal, but not immediately to the other one, rather after withdrawal continue to think. Without withdrawal but after the deadline expires the result of the thinking is no longer useful, with timely withdrawal you can still sign again with the same one if the result of your thinking says: he was actually the right one.

For me, there were even 5 and I am still active although everything is over because it’s fun.

And now? – what has your continued employment brought to informing yourself for the evaluation of your actions: would your decision be different today?

First, toxic spoke about the difference between Dacia and Audi, which is apparently much greater than that between Up and Citygo; even though I am not an Audi fan, there are worlds in between. Secondly, for example, details in the interior fittings in the Up are better than in the Citygo. You probably also believe the myth that an Oktavia has the same quality as a Golf or even a Passat?

On the topic: If I were you, I would resign. That is what the notice period is for.

First of all, thanks for the input. I still have a few days to withdraw within the deadline. I will meet a few people over Easter who have recently built.

It's probably normal to get cold feet with these amounts. But unfortunately, you only get wiser afterwards.

Offtopic:
Car manufacturers calculate in 0.1 cent (!). Why the Dacia is cheaper than an Audi, I can explain to you as well, but it takes too long and this is the wrong forum.
To make it short: Both are right, partly because of simpler and/or fully developed components and partly because of brand image.
The A3 has absolutely and relatively more margin than the Golf.
The same applies to Golf and Octavia. Although with those two, the lines blur. It depends on the engine and equipment. Btw, for all three, the floorpan including engine and transmission is identical. The Octavia is longer and requires more sheet metal. So what now?

Better stick to the topic of building. There seems to be more expertise than bar talk here.