I think allowing legends to create world events is a step in the right direction, but there should be some mechanic that encourages an "uneasy" cooperation until the very end.
Maybe the event is active only if the legend vessel remains afloat, and maybe the main treasure at the end is only available if the legend ship receives the final stone? Then, add a ton more ghost ships like flameheart, and now the rest of the vessels will be invested in protecting the legendary ship.
Second, reward the legend ship directly. Some of the treasure materializes on the vessel directly after placing the last stone, providing a base level of reward. Everything else you go after once the voyage is done would be an additional risk.
The way it currently stands, you stand to gain everything by sinking and stealing from the athena ship, and need to risk nothing. Like FoF, these voyages will only be done on alliance servers / dead servers, and risk alienating a large portion of the community who want more repeatable content, but don't want the constant PvP without feeling like they are being rewarded for it.
Better game design would be creating a dilemma for attacking ships. They want to sink the athena ship since it has a flag and some loot already, but if they created a mechanic like I am suggesting above, any ships that want to participate in the event would be incentivized to keep the athena ship alive, and so must strive to push them off the event without killing them to steal the treasures right out from under them. Or, board and steal the treasure that way / gain hand-to-hand superiority on the forts themselves.
The point is that the objective for would-be attackers is no longer just wait to the end and steal, it is now make sure we are in a good position when this event finishes to get as much treasure as possible from the forts, and ideally be ready to attack the athena vessel too. That takes a combination of diplomacy, positioning, skill, and discipline which make for a more engaging event, and gives the defending ship more opportunity to cut their losses if they feel outgunned, or even promise to cede treasure by only taking what materializes on their boat. They could lie of course, but that's part of the fun! You are the invader after all, attacking a legendary crew! So they can freely fire upon and sink you, but you are in a pickle because you don't want to sink them (right now at least), or maybe not at all. But good luck convincing them of that if you start stealing their stuff right away.
This is all what Rare originally had in mind for skull forts. They were pitched as these events with so much treasure that one vessel couldn't possibly get everything out themselves. It was going to be more of a free-for-all in which you get whatever you could get your hands on and make your escape. That is decidedly not how these events turned out. Requiring pirates to transport the loot on foot, making the forts too small to where you can easily fire upon one another, and locking all the treasure behind the final door encourages a winner-take-all approach / zero-sum game, which is more competitive, but ultimately a less complex kind of equilibrium. This actually provides less freedom by making the winning strategy apparent and binary. It is boring because it is predictable, and I predict that these voyages will suffer the same fate.