Is the EnBW advance payment too high?

hehe nicely said.. the service is by the way bad everywhere... after all, they sell electricity and not service
But please be aware that with your action you have neither won nor lost money.

That I ultimately did not make any money from it is clear to me, but I just don't like paying upfront. Better to pay afterwards.

But of course, everyone has to decide that for themselves. Some people are happy about a refund.

I have now significantly lowered the advance payment. In total, it is now only €170/month instead of the previously requested €400.

170 euros p.m. total is realistic in the initial phase, as you can't really estimate how things will go. In a year you will know where you stand and can adjust the kWh again (probably downwards). It was similar with RWE for us. A call helped there too (no 2x30 min waiting on hold). I explained back then that in the old apartment with old devices and no photovoltaics we had a consumption of 2,800 kWh household electricity and did not intend to increase this consumption massively. In addition, I gave them the readings of the two meters. That then convinced them and the price was reduced.

However, RWE did not help – we had already switched to another provider beforehand. But at least we were able to reduce the announced high initial advance payment directly (due to the construction phase).