nice update! like your setup very much, nice colonies!
for sleeper gobies: i have experience with diamond and banded sleeper gobies, the latter are better for a small tank like this as they also eat other foods and also nuisance algae more readily. on the other hand, they tend to ascend whith the sand in mouth so some gets moved on rocks, with your steep rocks i see less problems here than with the lps on the sand though. both gobies will bury them to a certain point.
i like my banded "phalaena" sleeper goby very much, keeps the sand sparkling and also eats red cyano, but not the green one. it also stays smaller than other sleeper gobies and is more beautiful in my eyes, not so dull white. i also have a sand sifting starfish in my 50g, but i doubt it could survive in a 30g with little open space.
greetings martin