: . . . I think I might fill the gaps before planing. Or maybe plane, sand, saturation coat, and then fill

The saturation coat will fill many small gaps. If you see resin dripping through to the interior, patch the area with a strip of masking tape, and use your roller or squeegee to go over the gap area again -- lightly.
the tape will dam up the gaps enough so that the resin from your satuation coat, even if thinly applied, will mostly fill the gaps.
what little depressions remain in the gaps will probably fill in on their own when you put on the glass.

You can just about guarantee that those areas will have resin filling them if you paint on your resin and then lay the glass cloth into it. Potentially messy, but when you run a squeegee over the glass cloth you poke it through the resin, forcing the resin to come through the underside of the dry cloth, and potentially eliminating, or reducing, a source of trapped air bubbles. The cloth lays over those gaps, and they epoxy resin in them hardens uneventfully.

PGJ