If you need an opinion from the other side of the DIY spectrum - this forum is full of guys who know what they're doing, and do it well, and have been doing work like this for a long time. They're 100% correct in that doing the work "right" means doing the extensive project, deconstructing and reconstructing the core.
Please don't accuse me of knowing what I'm doing. Sorry, that link I provided to the two core repairs is really long and winding. My description starts on post #72 from here:
Moyer Marine Inc. Parts and Services for the Universal Atomic 4 Engine
www.moyermarineforum.com
What I've learned from my repairs is that if you plan the initial cuts into the deck carefully, and are willing to apply KiwiGrip over the entire panel (both old and replaced parts of it), you can make a very solid repair that is decent-looking without a lot of cosmetic futzing. And it doesn't require much in tools - a grinder, an oscillating saw, some way of cutting G10 (a borrowed table saw, for example), and a pair of scissors - none of which are expensive.
You could use plywood as a replacement core, but I wouldn't.
I think people who do this repair from below are clinically insane.
The only trick to the use of epoxy in this repair is how to keep it off of everything.
Let me know if you'd like to see my fixes on City Island. I have a bunch of foam and epoxy cloth pieces left over that I may or may not eventually use, but would prefer to see at work in your fix if you can use them.