The Benelli Ethos is a beautiful shotgun. It’s a bummer it didn’t fit you right out of the box. I think you can self diagnose before having to call in a pro.
Here’s my method.
I shoot at a 12 inch round splatter target in the middle of a big sheet of cardboard at 17 yards. I use a full choke and #8 target loads to get a nice tight pattern, which will tell me exactly where my point of impact is.
I mount and fire the gun quickly, as if the splatter target on the cardboard is a flushing rooster (you may need to repeat a few times to get a good sample size). If the pattern is centered on the aim point, the gun is fitted correctly. If not, the point of impact doesn’t match point of aim and it’s time to shim the stock.
When you install a shim, it’s time to repeat the process to verify the result. Your gun should have come with instructions on how to install the shims.