TLDR? Read the last paragraph, not counting the footnote.
If the shear modulus we're talking about is in-plane, generated by twisting of the fins, then there is a big difference depending on grain direction. See page 21 (per the numbers on the pages) or 38 (according to the software):
https://kth.diva-portal.org/smash/get/diva2:1711342/FULLTEXT01.pdf
Notice that the in-plane shear modulus is much higher when the grain is */- 45 degrees than at 0/90. So if twisting is the problem, diagonal grain direction is best. If bending is the problem, then 0/90 is best. If bending is the problem, then 0/90 is the way to go. This may be tough to ascertain, because both are important in flutter. The same applies to the weave direction with fiberglass and carbon fiber. Plus, uni-directional is MUCH stiffer than woven, though in torsion you need two layers at right angles to each other. Different weaves have different stiffness. The straighter the fibers, the stiffer and stronger the laminate will be, at least in-plane. The difference in carbon fiber uni laminations will be much greater than with, say, woven fiberglass. I'm guessing that's because with the carbon, the ratio between the modulus of the fiber and the epoxy is so much higher than with fiberglass and epoxy.
If the outside layers of the plywood are at 45 degrees, then the glass should be at 0/90, I think. Unless you are using uni, when it could all be at 0, i.e. spanwise.
The bending and torsional stiffness of the fins will both be enhanced greatly if the root foil section is made a little thicker. The flutter frequency, and therefore flutter speed will be raised by reducing the weight, and therefore the moments of inertia, of the tips. If you extend the tips, use balsa or foam, though you might want to glass it for durability. Go easy with the glass, and blot the glass with toilet paper when you're finished wetting it out. The epoxy doesn't contribute nearly as much to stiffness and strength as the fibers do, as long as there's enough epoxy to restrain the fibers. If one were starting from scratch, I suspect that the fins could be made MUCH stiffer and somewhat lighter by using a lightweight core, thicker at the root, with composites on the outside. The airplane people typically make a spar to handle the bending part, but I doubt this is necessary for something so stubby.
If torsion is the problem, I suspect the fillets do a reasonable job of tying in with the body tube, assuming they don't crack away from the fin or from the tube. (What kind of surface prep did you use?) They will only help stiffen things in bending if the body tube is stiff enough. This, I think, is part of why people make fin cans. That and ease of positioning the fins. If it was me, I think I might just use glass and epoxy, and maybe a veneer core to build up the thickness to make a sandwich. The local bending stiffness of the tube is most important right next to the fin, and diminishes as you get away from it. So, if one was a complete weight weenie, starting from scratch, one might use a stack of sort-of elliptical laminations under the fin, each one skinnier than the previous. But that's probably too much trouble, and maybe unnecessary unless you are using nice, light glass.
I suspect that, with these stubby fins, the in-plane shear modulus of the plywood may be less important then the bending stiffness of the ply*. The local bending stiffness of the body tube is also important if you want the fillets to help much. If it was mine, I might sand down the fins so they tapered in thickness near the tips, sand everything a bit for better adhesion, do something to add thickness near the root, glass everything lightly, and add a strip of glass along the fillet, somewhat wider than the fillet. Note that the small bit goes on top of the bigger bit, so that the fibers go straight rather than going down over a step. If I had uni-carbon on hand (which I do), and I was really concerned, I'd make the strip along the fillet with it, going spanwise, which would also be in the circumferential direction of the tube.
* A lot of the emphasis on flutter is with airplane wings, which have a much higher aspect ratio than these fins. So torsional stiffness is really important for them, in addition to bending. With these fins, I suspect that bending stiffness is more critical.
