Any sane German employer will give you vacation days as a fraction of the time you work for them that year. So if you start on 01.07. you will get half the yearly allowance.
Any discussion about this will be seen as a big red flag by your new employer. It will probably lead to the exact opposite of what you want.
Your employer is allowed to not give you any vacation days in the first six month or during your probation period. What you read about "the full amount after six months" is the fact that if they give you no vacation days in the first six month, you are still entitled to take them afterwards, they are not lost, you are just not allowed to take them before.
Your employer is also not required to give you even the mandatory minimum amount, if you had vacation days before in the year. You cannot take on 12 jobs and just take your mandatory four week vacation in each of them to get paid for a full year of vacation. That is not how it works. Your employer is allowed to ask your former company, how much vacation you already had. If you already had 20 days that year with your former employer, you already had your mandatory vacation.
All of those things are details and laws and regulations nobody wants to deal with. The norm is that you get a fraction of your yearly allowance based on the time you are supposed to work that year. If you took more and then quit, nobody will bother, because it's just not worth the money to come after you. But if you show any signs of trying to cheat by haggling for more days based on laws and regulations with your new employer, you can be happy to get an interview at all, or if you pull that stunt during your probation period, to actually stay in that job. Nobody likes a person that is trying to cheat. You get your fair share. If you don't like the total amount of that share, find a job with more vacation days instead of giving everybody a headache sweating over laws and regulations.
So no, there is nothing you need to look for. You resign any point in time that is allowed by your contract. It does not make a difference, you cannot game that system. Not if you want a new job.
The only way you could game it is work longer with a company, then in the new year take your yearly allowance early in the year and leave after that. The old company will not bother if is somewhat reasonable (taking all of January and quitting in February is not, taking three weeks around Easter and quitting in May is technically a win for you, but nobody will bother to even calculate if you maybe got more than your fair share). The new company normally does not check whether you already had more than your share, they will just waive the paperwork and offer your your fair share of their vacation days.
So to summarize: there is no optimization. It doesn't matter. Unless you want to be really unpopular with the boss and HR before you even started your new job. Then you might optimize to get a free vacation day or two at the cost of not making the probation period. But that seems very counterproductive.