Develop agricultural land in a residential area Bavaria 10,000 m²

6. just by the way, I imagined lots of 500m2 plots

This is an impractical idea that ignores the market (and thus not least your sales speed). I recall my question about what kind of development the city would like to see. I would approach this more based on demand planning.

7. once again for understanding These development costs with roads etc. have to be paid 100% by the owner and I should try myself with local road construction companies to get a ‘deal’, there’s no way around it. Or I can recoup the costs as a road usage fee.

Effectively, as an interested party, you will also be the one paying economically in full for this matter. However, development is also a public task in terms of fees, and owner fees are not a toll; the issue is significantly more complex and you apparently need advice to an extent far beyond a chat in a forum. If you have serious need, you can feel free to contact me personally to put you in touch with the specialists.

The place is about 80 km east of Munich

You're almost in Austria then?
Tell me the district.

How realistic are the 400 euros per m2?

You are thinking about taking on the role of a developer. Basically, I think the idea is quite good. I just lack the belief that you can manage something like that yourself. Don’t get me wrong. I don’t know you, I’m just saying this because you ask such uninformed questions.
As already wrote, a forum is useful to broaden your horizons, but it can hardly help you with implementation.

I hereby offer you my help as well. But then less publicly. ;)

Why don’t you sell to the city and simply secure a few parcels for yourself?

I have no idea what kind of conditions you have, but an average citizen can’t just develop a field for a million and then build apartments/houses on it for a few million more.
If you are wealthy enough that the few million upfront isn’t a problem, then I would look for specialists on the subject.
Otherwise, I only see selling to the city or to a larger developer as a sensible option for you.

First, you have to talk to the city to see how they envision it. Because without the city, nothing will happen. And already at this step, you should have a capable planner by your side.

Oh yes: If you don’t know the right amigos in the right places, your plans will be over faster than you think. With the 4 million € [Bauland], you will have to let some people earn along the way. I believe you call that [Spezlwirtschaft] ;)

My ex-boss had back then roughly sold half to the city. The other half was developed as semi-detached houses. Half of those were sold as a property developer and the other half rented out.

Why don't you sell to the city and simply secure a few parcels for yourself?

Am I reading naive ideas about budget law or municipal economic law here? ;-)
Only a few municipalities are as (freely) capable of acting in this regard as they would like to be.

If you don’t know the right amigos in the right places

The relatively bigger problem is usually the existing connections between the local amigos themselves ;-)