liners are tough, because the curves are not able to be developed out of sheet goods. To get a sheet rock grade finish, takes a lot of putty and sanding
work. It is the reason why so many new boats come with a drop in hull liner, or have soft walls.
A hull liner built in with straight flat runs of plywood
is the fastest... works, but doesn't look as good but it is fast, and easy to get painted. Work boat style... if you do have to repair it you have to cut it out of the way.
Soft walls, or wood ceiling strips look good... and are very fast to install in Comparison to a curved hull liner like you are talking about. Soft walls and wood Ceilings are in all new custom boats, because what you are about to undertake costs more in labor than doing soft walls or wood ceiling strips. Even the ridiculous leather-like padded vinyl proprietary stuff still costs less than doing what you want.
However, if you want it flat smooth and painted with the foam as a small amount of insulation you may be on the right track. Getting from here to there takes a fair bit of time.
You will need to learn how to vacuum bag the foam into place. I would use a polyester based product like polybond rather than epoxy, as you will save a year of your life in the sanding
. Long Board sand the foam before fiberglassing it. From there, I'd put two layers of 1 1/2 ounce mat over the surface and let it kick... with polyester. Tough enough to take a bump, but no weave to grind through or show later. You can use peel ply if you like it, but you may be best off with 1-2 foot wide strips of it as a full panel doesn't stretch much.
Then sand it smooth with 80 grit. Take the shine off it by hand any of the low spots that may still exist. Wipe it down with solvent, then putty the surface with Adtech P77, or a high grade auto body filler.
From there you'll putty it once top to bottom with a screed spanning the lows. Sand it, then putty it once side to side and sand it. When it is all one color and evenly built out, you'll take a spray guide coat or dry guide coat and sand it with an orbital sander. Putty the pin holes, and then use awlgrip 545 primer as a sealer that will be a good base for most any topcoat you want to put on it. Household, and even automotive primer doesn't normally do the job over fiberglass
without adhesion issues some time soon...
Then switch over to an oil
base or latex house paint
to keep it easy to clean and maintain.
Fair disclosure: This is the sort of stuff you do once, and try your best to talk anyone else out of doing it in the future. The entire panel is a low spot. Fairing the outside of the boat is one thing, because the board rides over it. Making a concave surface look good without lumps and bumps takes a bit more finesse....
Wood trim and joinery covers a heck of a lot of ground quickly. It looks fiddly, and you need a tools and work space... but a set of cabinets hung in place would save you a lot of itching.