That's tough work you're doing there. I hope you are somewhere that isn't too hot!
I'm probably not qualified to weigh in on this, but there are a lot of folks that are on this forum so hopefully they'll chime in. More information from your end may be useful and there are a lot of details with this project
that come to my mind after looking at the photos from the best way to join the new core/reinforcing to the old, to preventing the rot
from reoccurring, etc.
From what I can see, using a rotary cutter
of some sort to define the edges of what you are removing and then using a chisel/prybar and similar to remove the biggest chunks would be where I would start with the removal
and it looks like what you've been doing. From there I would get after it with a grinder with 5 inch coarse sanding
discs, like 40 grit or so. That should make a great mess and suck a lot but should get you down through the old epoxy to the original deck for the most part. Smaller tools can help you get to GRP or ply in the tight spots and a shop vac and/or grinder with dust collection can be very good for this type of work along with great dust shielding and ventilation in the boat.
From there I'd consider how you will join the new layer with the old, ie should you just butt the two together or feather the joint in some way to avoid creating a flex point at the seam between the panels
. Beyond tapering the wood edges for mating you may also have luck with glassing the seam to spread the loading, etc.. The overall structural situation of this part of the boat has to be considered in the repair but you may already have a good plan for all of that.
How to hold the new layer into place will in part depend on how firm the deck is above the repair, if it will be refinished etc. I bet there will be more good ideas but Drilling and screwing into the new panel from above may be a good option or it may not work if the skin is too flexible or if the deck isn't to be refinished. Using props from below can work really well if they are able to apply pressure uniformly over the panel so that it doesn't distort or pull away in low-pressure spots. 1/2 ply is pretty stiff though for the type of props that I have in mind so your 2x4's and jacks will probably be easier. You will want to spread the load of these supports though so that you get full and even contact between the deck and the new reinforcing layer. Also, if there is a lot of curvature and the deck is flexible you have to watch for the new panel pulling the deck down flatter than it was before and a solution if that is likely may be to ad multiple thin layers of reinforcing which can be laminated in place so that they take the shape of the deck and reinforce the deck shape vs. fight against it over time.
Good luck figuring it all out and I hope you get to move on to easier projects soon.