Ongoing additional costs - Realistic?

Especially with household contents, I would look very closely at what value is actually being set.

I strongly suspect that quite a few are underinsured, looking at your premiums.
I have an annual premium of 180 EUR for my condominium. Some here pay a bit more than half of that for a house!
Or are there overlaps with the building insurance and therefore less coverage is needed?

I thought the same. For household contents, we pay €19.80 per month, so about €238 per year. We have underinsurance protection included.
Building insurance for us is €38.40 per month / €460 per year.
Good coverage is important to me. The event occurring is unlikely, but if it does happen and you then realize you are underinsured, you can quickly be financially ruined.

So I really can't make sense of why your costs for residential buildings are so high. Sure, the location plays a role, among other things. I just entered the current situation on a comparison portal and get about €130-150 as an annual premium for my 100 m2 house including glass breakage. With a tariff rating of excellent at €150. Applying the same situation to my previous address in Dresden makes no difference. If I change the living area to 150 sqm, the value goes up by €50-60. Each also including drain pipes on or off the property. The waiver of underinsurance is included in each case. I have also never heard that you have to explicitly book that if you let the insurance sum be calculated automatically. Unless you specify a certain insurance sum, then I think the waiver can be associated with that.

Also quickly did the test for household contents too.... costs about €50 per year and with the larger living area about €20 more. The insurance sum is stated/suggested as €65,000. I don't know what you've hoarded in your places ;-)

in each case the waiver of underinsurance is included.

both, that is building insurance and household contents have no deductible.

We are definitely not underinsured. However, we have only insured what is necessary. For the residential building, we used to have around 300 and now they increased it to 320, which was too much for me. We now have coverage without flood, avalanche, etc., and with a 1000€ deductible, which makes the premium significantly lower. We really don’t need natural hazard coverage for our situation, and the deductible has noticeably reduced the premium. If the entire house burns down, we can surely manage the 1000€. We only insure against what would financially ruin us. Fire, storm, etc., that’s important and we have that.
Our household contents definitely cover everything I want covered. Glass breakage, for example, is not included, but our bikes are. The coverage amount is high enough to replace the entire inventory.
And it goes on like this. If you want coverage for many contingencies, choose little or no deductible, or if you live in a flood zone, then of course costs go up.
We have Internet and phone with O2, I believe, for 29.99€ per month, we don’t need more. I’m also not willing to spend 70 or 100€ per month on something like that.

I would be cautious there. What matters is not the price given to you by a comparison portal, but what is actually included in the contract.

For example, with us, coverage includes exemption from contributions in case of unemployment and a currency update guarantee.
My experience with such comparison portals: the real problem comes when you actually want comparable services to those supposedly more expensive insurances.

So far, the comparison portal has really only helped me to find cheap electricity providers; otherwise, I mostly had the impression of bait offers, and when you actually had a concrete offer made, it suddenly became more expensive.

Glass breakage is not included there, for example

How is it with the photovoltaic system? It is included in my glass insurance, that was important to me (well, I took out the policy after the severe hail here, we were also a bit shocked at what all got broken).