Solar possible, how do I proceed? Help, tricks and tips

Addendum: You should be quick. The 52 GW cap is looming and then it could be that there will be no more subsidies and photovoltaics might no longer be financially viable.

So you currently have a functioning solar thermal system (hot water producing panels on the roof) If possible, take it down and fully cover the roof with photovoltaic As mentioned, give it gas!! The 52GW cap will probably fall in August / September. Then there will be a follow-up regulation that nobody knows yet and is expected (looking into the crystal ball) to be worse than currently (feed-in tariff still guaranteed for 20 more years, your system pays for that, plan without battery, if you want it’s possible with one as well. Just be aware that the system will pay off later because the battery storage eats into the photovoltaic return and is still an (expensive) hobby. There is a special forum for that, they will gladly help you there ...

so you currently have a functioning solar thermal system
(hot water generating panels on the roof)

If possible, take it down and fill the roof with photovoltaics

As mentioned, go for it!! The 52GW cap will probably fall in August / September.
Then there will be a follow-up regulation that nobody knows yet and expectedly
(looking into the crystal ball) worse than the current one (feed-in tariff still secure for 20 more years, your system pays for that, plan without battery, if you want you can include one. Just be aware that the system will pay off later because the battery storage eats into the photovoltaic yield and is still an (expensive) hobby.

There is a special forum for this, they will gladly help you there...

In this context, I would like to ask the following question.
Originally, our installer planned an installation in November due to workload. Until then, much can happen regarding the cap (fortunately, we have a clause in the contract that entitles us to withdraw from the contract if the cap takes effect before commissioning(!)), so every day sooner can only be good.
Now it happened that they recently were on the construction site and wanted to install on the roof. Our architect sent them away again at first. She justifies this by saying that although the roof is covered, since the dormers will only be finished when the [düngender] are in place, it could be difficult for the roofer if the photovoltaic modules are already on the roof.
That sounds reasonable at first.
Nevertheless, I think it can only be good if those things go on the roof as quickly as possible, so that as soon as the electrician has been inside, the system can be connected to the grid.
Or am I missing something?

OK, I will try to have a conversation tomorrow.

What I understood is that feeding in is better than using it yourself (including battery). Are there any other subsidies or promotional loans?

Not quite, using your own electricity directly is basically "the best".

OK, I will try to have a conversation tomorrow. What I understood is that feeding in is better than using it yourself (including battery). Are there any other subsidies or promotional loans?

It varies from state to state. Just google Hessen and Photovoltaic storage promotion.