"Toller"
I have epoxy (and linseed oil and polyurethane varnish) but hesitate to
use it since it is not UV stable.

Almost nothing is UV stable, including paint, wood, epoxy, varnish,
polyurethane, and wood.

However, sealing plywood edges with epoxy, lightweight fiberglass (with the
appropriate weight fabric and a curve that it can take), covered with primer
and paint, is as good as you can plan for. As long as you maintain the paint
for appearance, it will probably serve to preserve the epoxy & fiberglass.
I figure that a decent job of it, aside from direct scrapes and dings,
should be good for 5-10 years of regular use.

If you're looking at the leading edge of a kick-up rudder that meets the
ground routinely, you'll need more than one layer of glass, and regular
touch-ups of paint.