Do clean off the old white paint and try to get a”key” into the GRP, probably best done mechanically, don’t use chemical stripper.
Do use epoxy resin and woven cloth (not polyester resin and Chopped Strand Mat).
If you laminate a further layer or two of glass under the cracks you will strengthen the thwart, but if you can introduce a spacer (eg foam strips) so you end up with a sandwich, it will incrrease the strength many-fold. In fact why not laminate a piece of wood under there so you can screw stuff onto it (as in Dave’s boat)? I have some cheek blocks under there to divert controls to the helm.
Mike