Property purchase & broker behavior: Is this normal?

Can we still somehow save real estate transfer tax by declaring the inventory within it?

Maybe just generally set it at €10,000.
But I think you save significantly more if you manage to get the clause that the house must be emptied through. Otherwise, you’ll be paying an arm and a leg for all the leftover junk. And/or the clause that all movable items on the property transfer to your possession, to avoid edge cases where the former owner still stores belongings there but doesn’t collect them. However, you aren’t allowed to just dispose of them either.
Anyway. Above these detailed optimizations, my opinion remains the same: better to swallow the disadvantages than to scare off the seller if you really want the property.

Can we still somehow save real estate transfer tax by declaring the inventory therein?

Yes, this is especially often done with fitted kitchens. Just don't overdo it so that the tax office examines it more closely and possibly wants a detailed list of the inventory. Old furniture doesn't have much value. For example, an awning may also be deducted, as has been accepted and decided by a court. (See FG Cologne 8.11.2017, 5 K 2938/16)

Anyway. Overarching these detailed optimizations remains my opinion to rather swallow the disadvantages than to scare off the seller if you really want the property.

That's just our concern.
He seems to be more of a difficult type or to know his position of power.

Then better to set the 10,000€ – that probably doesn't matter to the seller?
I think he won't like the clearing out. He probably wanted to get rid of it as quickly as possible.

He seems more like a difficult guy or someone who knows his position of power.

You can make your changes in the draft, and if he doesn’t want them, then it can just stay that way.

What is stated in the contract is one thing... whether, for example, he actually empties the house is another matter. Taking the whole thing to court or shifting costs around doesn’t help at all.

I also had a difficult guy. He canceled on several families right at the notary appointment and stood them up. No idea why he signed with us. Oh yes, of course he left all the household contents and simply got on the plane.

oh wow
Did he then arrange several notary appointments in parallel or what?

No, one after the other over at least three years.