I use a Dremel for creating an even gap between the stitched panels.
I've used both the rotary blade and the little router bits as below.

I find it a really useful tool for this kind of thing and the sanding reels are handy for tight spots too.
However, I think the Dremel is a bit "light duty" for cutting entire ply panels out.
Part of the reason it works so well for trimming the edges of stitched together panels is that, for the most part, you're only slicing away
a whisker of wood to open up a gap between two panels that are butted together, usually on an angle.
On the rare occasions I've used it to cut right through 4mm marine ply at 90 degrees, it struggles enough that I think it would take a heck of a long
time to make a continuous 6 or 8 foot cut.
My opinion is that the best combination is a good jigsaw with a fine metal cutting blade to make the main cuts, (although that Bosch Multi looks pretty nifty) sand down to the line, stitch the panels and create a nice consistent tack welding/filleting gap between the stitches with the Dremel. Once the tack weld has set, cut away the stitches and open a similar gap where they were and complete the filleting.
John.
1 Cape Fear Sit In....a couple more planned
2 Laker 14's
1 Paddle board
"People who don't make mistakes don't make anything"