: I'd prefer not to place a FG tape patch on the outside for cosmetic reasons -
: thinking of sanding down the inside ridge, applying two or three
: overlapping layers of FG tape, then sanding the outside indentation (it's
: pretty shallow) and filling it with a epoxy-silica mix. If anyone has a
: better approach, or sees any .

A patch from fiberglass cloth will be nearly invisible. Any filling on the outside will be a big blotch.

Fiberglass tape is not the material of choice for repairs on flat surfaces. Almost any auto parts shop will sell you a yard of glass cloth.
The plywood cracked, and the parts are not falling back exactly into their original positions.
You may need to do a little cutting to get the plywood to spring back into place. A carpenters utility knife will do the job if you make many shallow passes on the same line. There is no kerf, or removed wood, except for splinters which may fall off, and which can be glued back for cosmetic reasons.

After you pull the pieces back into place, sand back the area around the wound, tapering or 'feathering' the sanded area for two inches to each side of the damage.

Cover this area with resin and lay on a patch of fabric which extends past the sanded area. Wet out the glass, put on a fill coat, sand down the borders where the patch overlaps, and when everything is flar and smooth, put on a final coat of epoxy to cover all the work. Sand and varnish as usual after a few weeks.

On the inside you can make a patch of 2 layers of fiberglass to reinfoce the damaged area and restore strength, or use plywood.
It doesn't need to be as pretty.
Hope this helps.
PGJ