I made some improvements, and now it works on the beam, good enough anyway.
I made a much better fin - it's longer, and more fin shaped. It's sealed with epoxy. There is a hole drilled from the top, with a bolt into it. The green rope is epoxied on to reinforce what I expect is the most stressed part. Okay so I spent another $5 on rope but only used a tiny part on this. Shown next to the previous fin. Also I brought the control lines in closer to the pivot so it is more responsive.
Because the fin is more powerful, it broke pivot at the other end of the pole, so I had to make that better two. The second attempt worked.
It took a few tries to get this to all fit together right, but it works better now! I sailed back from mahurangi with 15-20 knots on the beam, to top of motutapu, no problem. Then today back to auckland with 25 knots to windward. Can tack by just turning the wind vane then, handling the sheets! (just don't trip over the power lines to the tiller) Maybe I can rig up a remote adjustment to the wind vane.
Oh yeah, the windvane wobbled a lot in 25 knots, maybe need a way to reef it?