Deficiencies shortly after purchase of used car, how do we proceed?

Hello, my girlfriend and I have entered the world of the Seat used cars, in the middle of August we bought an Ibiza (automatic), 13 years old, 130,000 mileage. The car did not start already on the yard of the dealer and needed start aid, it was called “he stood for a long time”. Unfortunately this continued and he needed almost always start aid – battery was changed (at his own expense). In mid-November then warning lights EPC and Symbol ABS, car could start, but not At the dealer a throttle flap was replaced (at least warranty). Now at the end of November the LED for the exhaust system, i.e. lambda probe or similar, said immediately (without seeing the car) “wearing part, repair costs they carry themselves”. We have now almost three months, have driven the car itself only approx. 1,000km and are already a little frustrated. The dealer on the one hand has good reviews, on the other hand we are still waiting for the letter I confirm that the repair of the throttle flap was a guarantee case (we would like to present to the insurance company because they have borne the towing costs). We are still absolute beginners, almost the first own car. Do you have an idea how / where we can start to “fight” against the dealer? The car is still checked the week by the dealer, I am sure he will say “wearing part, costs xy”. And did we dare not with the Ibiza? Are these many things Is it even normal? Actually, we had read only good things before. Look forward to your experiences and maybe ideas.