I finally got the last heirloom ring on my shipyard table.
So I have seen all the rare shipyard missions across multiple toons. I can now stop running the shipyards on a number of toons. My mains will continue to run it until one of them sees all of the rare missions for the Exploring the High Seas achievement. The amount of effort necessary to keep the alts running the shipyard isn’t worth the occasional gold mission that pops up. All alts are in baleful (or lfr) gear or better, so the gear missions are unnecessary too. The Alliance main is going to see at least a 2-piece normal-tier bonus, as well as a heroic item or two, before the expansion is over.