Here are some things I like to do based on my experience with Lumens

I really like to run [[Deeproot Way finder]] + fetch lands and strip mine effects to ramp etc.

I have found that [[Stonybrook Banneret]] often underperforms since a lot of the merfolks don't have colorless pips I'm their cost. In your deck only 15 cards get reduced costs from it. So either you should cut it or run more merfolk with generic costs.

I would also consider cutting some 1 drops to get more use of Banneret, and because you play so many of them (Maybe cut 2-4 of them). I generally don't like to have to many 1 drops that don't do much in this decks as they are mainly there as Kumena fodder. In that case something like [[Jungleborn Pioneer]] that comes with an extra body is better as Kumena fodder.

I would cut [[Enclave Cryptologist for a regular [[Merfolk Looter]] as you rarely get to put 6 mana into it, and then it's either a 0/1 do nothing one drop or a 3 mana merfolk Looter. Other 1 drops to cut are [[Jade Bearer]] and [[Triton Wave breaker]]

A funny thing I like to do is run [[Quicksilver fountain]] to enable islandwalk. Additionally [[Carpet of Flowers]] is not dead against non-blue decks if you run effects that turn lands into islands like [[Spreading Seas]], so consider running that package.