Plot on a slope in the Munich outskirts - how to decide?

Resale is always excluded when it is sold by the municipality and presumably also when it originates from private individuals and the municipality has developed it. The only option is to return it at the same price, and you have to bear the incidental costs yourself. There is no speculation tax, even though many keep claiming it. You have to tax the profit as income if it was rented out. That means in your case it is tax-free twice.

I have looked at the guideline. Resale is not explicitly excluded. The municipality only has a right of repurchase. Does anyone have experience with this – does the municipality often use this option in a potential resale?

I have absolutely no intention of reselling the property, but if raw material prices do not recover, one would have to think carefully again about whether to afford a construction project.

Consider whether you really want strangers in your house (Einliegerwohnung). For me, that would be a no-go. You can think about a separately usable living area (possibly for elderly parents, a nearly grown-up child, or the like), but which is currently available to the family. A separate, rentable (and subsidizable) Einliegerwohnung must meet certain requirements and therefore costs more. I don’t know why everyone is always so obsessed with an Einliegerwohnung. It takes away living space and you bring strangers into your private area. You have to like that... That you make a profit from it or that financing becomes easier – no one here has really been able to prove that to me so far.

While you are waiting for prices to relax (by the way, are you doing that alone, or is Godot waiting with you?), further increases due to building regulations may be added, and rent still has to be paid. A garage is privileged in terms of boundary, but using its "basement" as living space is not.

That anyone makes a profit from it or that financing becomes easier with it – no one here has really been able to prove that to me.

Financial mathematics is too much of a yawn topic for me to have the necessary enthusiasm to prove (that it is more likely for a camel to go through the eye of a needle than for a small landlord to get rich).

Think about whether you really want strangers in your house (granny flat). For me, that would be a no-go.
You can consider a separately usable living area (possibly for elderly parents, a child who is almost grown up, or similar), which currently is available to the family.
A separate, rentable (and subsidizable) granny flat must meet certain requirements and therefore also costs more.
I don’t know why everyone is always so obsessed with a granny flat. It costs living space and you bring strangers into your private sphere.
You have to like that...

That it makes a profit or makes financing easier – no one here has really been able to prove that to me yet.

I don’t have a finished concept yet and therefore I am orienting myself by how other people in the area have done it.
The granny flat would definitely have its own entrance and would not be directly in the main house. On the other hand, you probably wouldn’t get a fortune per month for a small granny flat even in the Munich suburbs, and somewhere the investment has to pay off. But I haven’t looked into it in detail. Even if it would only be rented out externally in the medium term, the children will still be happy about a small apartment one day.

one should think carefully again whether one wants to afford a construction project

That is the wrong order. Whether you WANT to afford the project, you know before all other steps – especially before purchasing the land. And up to which limit you CAN afford it, your financial advisor will tell you in one of the first steps of this undertaking.

Waiting / dawdling is currently one of the most expensive luxuries in house construction, and eine granny flat is never an investment (but rather a zero-sum hobby, which in my opinion you can lump together with KfW40). Where a double garage is dreamed of, there is always enough potential to get started. Its share of the budget alone is a huge tidal shift in the war chest.