On that note, I will definitely suspect the tires at first because if tires are not balanced 100% properly this is an exact issue you will face of vibration or wobble after certain speed and then after more higher speed this vibration goes off. I strongly suggest going back to the guys who have installed tires and ask them to balance tires properly. This time stand on their head and make sure to see 0 and 0 on both sides of each tire even 5/10 and 0 will cause an issue, so force them to show 0 and 0 on their tire balancing machine for each tire.
If they charge money for fixing their problem, better go to some reputed tire shop and get all 4 tires balanced like I said, it's gonna cost less than 60-80 dhs at max