Defected odometer when buying used cars

Hello everybody, I’m very fresh here and I have a somewhat unpleasant situation to describe. I bought my first car today or yesterday, a Volvo 850 GLE. Now, during the course of the day and especially during the transfer journey, I noticed some defects that were not discussed before or not recognizable. The car is so good, the engine runs round, the car rusts not unusual. Now on the actual topic: the car was advertised with 300 .400km mileage, well. For the tour and test drive, which unfortunately was a little short, in about 5-10 minutes, much to me that the mileage was already close to 345,000km, I also mentioned that and considered during the price negotiation. What I did not notice was that the tacho is broken and no longer runs. This was only noticed today when picking up on the highway, after contract had already been signed and car was paid. In the purchase contract was other of course, I should have noticed the thing and the contract should have been completed more thoroughly, but I would like to point out that we all make mistakes and, since my first car, I also have little experience. In any case, I noticed afterwards at home that the current standing mileage is exactly equal to the mileage of the last TÜV report of September 2018. Furthermore, it has now emerged that the car already has 5 previous owners, in front of me, but the seller assured in the advertisement and in the presence of my companion that there are three. I was able to see the vehicle letter and bill for the first time at collection, but the contract was signed one day before with down payment. In addition, there are still small defects like a defective thermostat, since the car is generally good. However, I would like to know if I have a chance that the defective odometer is a reason to annul the contract or to receive a discount (repayment). Perhaps someone can help me here. With kind greeting Marvin Reinhardt