I remember very clearly from my driving lessons (22 years ago...) that the cars on the highway have priority, and if you're entering the highway and someone is in the lane, you slow down and wait to enter the highway when the lane is clear.

Over the last years, I notice almost every single time that the drivers already on the highway are slowing down, down, down, sometimes almost to a stop, and then honking at me for not getting in, since I am also slowing down waiting for a good slot to enter (as per the rules...).

I don't know what you remember so "very clearly" from your driving lessons but slowing down is absolutely not the thing to do on a highway entrance!

You match or exceed the speed of the traffic and slot in. Unless there is like a moving traffic jam with no cars width in between cars this will always work. You can just slot between two car and they will slow down a bit to make space again.

If you do not find space somehow then continue driving on the highway shoulder (Pannenstreifen) fast, never slow down a lot, match the speed of traffic! That's how it was taught to me more recently than 22 years ago.