All of that is incorrect.
You can check the GEMA fees here: https://www.gema.de/portal/app/tarifrechner/preisrechner - I had it calculate the fees for a 4-hour concert with 100 seats and an entrance fee of 5 Euros, and the total GEMA fee is 31,35 €. If all seats are filled, that is less than 10% of your income. Certainly a lot of money, but a far cry from "all of said fee or even more".
Secondly, if you are not a GEMA member and all the music you play is non-GEMA (and written by yourself), you don't have to pay anything, see here for example: https://www.anwalt.de/rechtstipps/gema (central quote: "Der GEMA-Pflicht unterliegen dabei alle Musikwerke, für die sie die Verwertungsrechte erhalten hat", translation: "GEMA liability applies to all musical works for which it has received exploitation rights.")
I don't like the GEMA either, but let's stay with the verifiable facts, please.
sigh If more than 7 seats out of 100 are filled, you will make a profit. Happier?
I'm not going to comment on your second paragraph, since it doesn't contradict what I wrote. And if I were a GEMA shill, I probably wouldn't provide links for my claims, but you do you.