Island Sanctuary feels intentionally slow and inefficient.

[Absurdity]

My only issue is that it feels pointless to make any monuments or whatever they are. While those are cool they don’t really add anything important to the island or change anything.

They absolutely do something. But as always with this game it is unreasonably vague about what they do.
Landmarks increase your maximum Groove. Groove is a buff in your workshop menu that gets higher the more things you sell consecutively in your workshop and it increases how much Seafarer's Cowrie you gain from it, 1 point of Groove = 1% more currency gained from sales.