I heard the wings failed at the end of the ply spar doubler.

All your suggestions sound good. On my Ginat Stinger, I added some fiberglass tape over the spar extending 6" out from the doubler. I have flown the stinger hard for several years, no failure. If I had carbon fiber, I would have used that.

I did get a crack in the fuse at F2 on a landing I hit a deep rut. I had added some fiberglass to the inside, but needed more.