Hi Ellio, a couple of ideas - one possibility could be to cut away some empty space and then use Consruct > Blend to fill in that space with a blend surface.

Another possibility could be to avoid "baking in" any rounded corners in the starting outline where the rounded corner has a tighter radius than the fillet. Make those areas extend to sharp points and then maybe let the filleter round off those at the same time.

Guitars are a pretty difficult type of model. I think there are some tutorials you can purchase for it here: https://3dcncguitars.com/ that might be good to check out for some strategies.

A little later today I'll try experimenting with your model a bit and see if I can give you any other ideas.

- Michael