Thin is better.
There are 2 basic types of vinyls. Cast and calendared. Cast is when the vinyls is more or less poured into a mold, while calendared is pulled like toffee into shape. Cast is better, with less shrinkage and usually longer life. The calendared vinyl seems to want to return to the shape before it was stretched into a sheet.
So when they give you thin vinyl it is actually the best you can get. Seems like the thinner it is, the less it shrinks. I have found that vinyl shrinks so much over the years, then it either cracks and splits or comes off the adhesive. So the less it shrinks the better.
There are some really thick decal materials made for things like motocross bikes. They are very thick, stick to anything, but I am not sure about the longevity.
If you want something a little thicker, and cost effective, there are some new high end calendar vinyls that claim long life and are a bit thicker. Oracal and Avery are 2 makers of vinyl that people use.
edit: I see you are using a 3ml vinyl. That is most likely calendared, so it is not the best there is. Oracal 3651 is a great go to calendared vinyl. I like the 2.25 ml 3571 air release from Oracal.
