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Exchanges have limits based on diverse criteria, which is different from one another. For example, Coinbase Pro has a withdrawal Limit of 10k$/day (under certain circumstances, the limit may be increased). Kraken, with an Intermediate account seems to allow for 100K$/day, with a 500K$/month limit (Pro account has higher limits). You’d have to study a bunch of trustworthy exchanges when the time comes, and compare conditions at the time.
As @OmegaStarScream indicates above, I would be more concerned with having issues with the bank, and in a second order of concern, getting things clear for taxation, being able to demonstrate the origin of the income (and it’s not simply "from the sell of BTC", but rather more being able to trace when the BTCs were originally purchased, if, when and how trading was involved, etc.).
<…> you could use ATMs (Germany don't have these but they're available in most of the other European countries) <…>
This list may not be up-to-date, but it seems to list 34 BTC ATMs in Germany:
https://coinatmradar.com/country/80/bitcoin-atm-germany/Not all allow you to sell BTC, and there are limits specified (in some cases) of a couple of €K before needing to use ID.