It is important so show gratitude in a respectful way and you can show this in many different ways.
Thank you very much and Thank you so much are both correct and there is no major difference between them.
Thank you so much has become popular over the last few years. So is a little bit stronger than Very and people use it to show more enthusiasm/gratitude.
Some people feel that “thank you so much” does not sound genuine or sincere. I also agree that sometimes it does not sound genuine because some people use it a lot and in my country, people only use it when it is really necessary.
I believe that younger people especially from the U.S.A use thank you so much and there is nothing insincere about when they use it. Remember, English from North America is becoming popular all over the world so it is also common to hear thank you so much used a lot in the U.K and Ireland.
If you and to be safe, neutral, and more reserved, then you can use Thank you very much.
If you really want to be grateful and enthusiastic then thank you so much is better.
So and Very are both intensifiers and to learn more about intensifiers you can read more about them here.