That's a very attractive bench and a sad story about it. perhaps some part of this may help.
I built 2 of these garden gates 15 years ago with red oak. I treated them with a wood preservative, zinc naphthenate. I was working with the stuff at the time. It was a bit of an experiment. They have held up extremely well. Even the wood in the corners most susceptible to moisture damage is still uniformly sound. There is no other finish on them. I live in a much colder climate. Also, they are porous red oak, not white oak, so they soaked up a lot of the stuff. I think Louisiana Pacific sells this in small applicators to treat the cut ends of their engineered wood siding. Cuprinol is a commercial preservative with similar chemistry, but I think it is copper rather than zinc based and can discolor the wood. If the wood will absorb it, working preservative into any cracks should help. These are toxins, not repellants. They kill what eats the wood after it takes a bite not before.
It's been my experience that gluing to end grain causes more problems than it solves. Any gap becomes a point of water entry, and then the glue line is a trap rather than a barrier. In architectural pieces, metal flashings are the common way to cover exposed end grain. I cut and folded thin aluminum to cover the exposed end grain at the tops of the 3 brackets that attach this small door canopy to the wall behind it. This is trickier on a piece of furniture, but I've seen copper bent and hammered to cover the tops of posts.
I subscribe to the old theory that knife finished end grain weathers better than sawn or sanded end grain. After I cut the white cedar fence posts to pyramid tops, I cleaned them up with a block plane; and they haven't fuzzed up since the fence was built. The curved ends of the brackets and rafter tails in the door canopy were finished on a shaper with a jig, and they remain as smooth and unchecked as the side grain. Not sure how you worked the curved ends on the bench posts.
Finally, preservationists generally use epoxy to stabilize deteriorated wood. Forest Products lab did a study that identified epoxy as the most effective long term moisture barrier on wood.
I hope you find a way to enjoy your beautiful bench for many more years.