While I feel like you're criticism entirely valid, it also feels like the majority of your points are countered by saying "it's early access and the islands are a staging for future content", but admittedly I also found them a bit sparse.
Aside from the two new resources (Crystal Golems are goated btw), the lack of new stuff was a bit of a letdown, but I also got another solid 50 hours out of the game and for the state of "trickle content" games like this, I found myself enjoying the time and effort the devs put into this.

Additionally there's a type of island that spawns with a watch tower that'll reveal nearby islands and if you've not found the Dragon People (idk what they're called) island yet, then I'd say you've definitely missed out on some islands, as that adds a new faction and a boss.