I have a very simple buttercream I usually use. It's
2 1/2 sticks unsalted butter
2 cups conf sugar
and mix

It's really good, a bit sweet. What can I do to make it less sweet??

That BC recipe is going to be sweet no matter what you do to it. You can try adding some salt, but I think that just enhances the flavor. You can try adding a dash of lemon juice too. You do not want so much that you can taste it. Or you can just try adding less sugar.

Because you are new here, you should experiment and try some of the other recipes posted in the recipe section. There are some really great ones and besides it is best to have a variety of frostings instead of just one. IMO there is not one end all be all frosting to use for everything. The frosting should complement the cake, not be the same old thing again and again.

And lastly anything made with sugar, especially PS it going to be sweet. I can't comprehend the concept of taking something that is sweet by nature and trying to make it less sweet. The only way I can think of is like I said before, less sugar means less sweet. Just like salt. Less salt makes it less saltier.

Try using SALTED butter. Salt cuts sweet.

A couple of tablespoons of flour works well too.