If I get pre-settled status, does that mean I will have the right to work after January 2020?
With a combination of low salary and issuing dividends your personal tax liability will be something like 5% on £50,000 of income
You can stay in the UK for a further 5 years from the date you get pre-settled status and the deadline for applying is 30 June 2021, as long as you were living in the UK before 31 December 2020.
The settlement scheme is basically just a mobile app for you to take a selfie, scan the tag in your passport using NFC and answer a few questions like "Are you currently in the UK?", "How many years have you been living here?", "What's your NI Number?" ..they will then check your employment records electronically.
So apply after you get your NI No. by letter and your limited company is setup. You will usually get a decision a few weeks later which is just an email, explaining how you can prove your status online to others (like landlords or employers) if needed.
Regarding taxes, google for "optimal directors salary and dividends uk" and you will find loads of articles by accountants explaining the how and why in detail.
Living in London might just be cheaper or similar as Berlin after tax savings. Because I believe in Germany, taking profits out of your company will incur an additional 25% personal tax whereas in the UK, dividends are only taxed at 7.5% up to a threshold:
https://www.gov.uk/tax-on-dividends
But don't take my word for it, the German tax system looks like a nightmare.
If you have any other questions let me know.