Sure, I have preferences about the positions I applied for but due to the circumstances, should I just sign the first that enters? Or should I wait (and maybe even miss opportunities)?
There is still time until mid of Jannuary, so I personally wouldn't jump on the first bus that comes my way if it isn't ranked under my top choices, especially when having applied to that many positions and still waiting for multiple final offers.
Some might argue that one can still leave within the probation period if some better job-opportunity comes along, but I personally refrain from such tactics.
If you have sufficient financial backup, why not wait a month or two and continue to apply and also enjoy your short time being unemployed (when having worked a couple of years it's sometimes beneficial to have a break of one or two months).
Since you mention Germany as the country you are working in you are entitled to unemployment benefits if you have worked for a certain amount of time which will make your job-hunt more relaxed in terms of finance.
From my feeling, one shouldn't let a company wait for more than a week with the signature, or?
Hiring-processes can sometimes take months, so if you receive a written and signed offer it's completely ok to let them know that you need some time for consideration, but not more than one to two weeks (that's just my estimated gut-feeling and can vary from one company to another).
If you are still in the interview-process with multiple companies, then there's no need to make an immediate choice and you could still continue to hunt for more/better opportunities.
Again, my advice is the following: If you have a sufficient financial backup, take your time with finding your next gig and enjoy a little (emphasis on little) break from the worklife. A rushed decision can lead to ending up being underpaid, undervalued, unhappy and then you might have really missed out on a gig that suits you much better in terms of income, commute, work-life balance, company-culture etc.
As per comment of @Simon, you could also call the "more preferred" companies once you have an offer from a "less preferred" company, and ask them if it makes sense to wait for them.
Also @Joe has a good point that you should only hand in your notice when you already have a signed offer in writing - I guess that's a lesson learned for the future.