It’s always worthy to upgrade your harbor to the max level you can. I didn’t know that until I figured out the points-to-chests changes in each level. And this is based on “don’t get all the chests, focus more on the quality of chests, instead of number”.
When your harbor is level 4, you need 220 points to get 2 blue chests, 2 purple chests, and 1 god-like chest.
But when your harbor is level 5, you need 230 points to get 1 blue chests, 2 purple chests, and 2 god-like chest.
It seems you need more points so the cost is also higher, but think about if you want to get 1 god-like chest, the cost at level 4 is to get 220 points, but at level 5 you only need 180 points. Although by only get 1 god-like at level 5 you have 1 less chest compare to level 4, but it’s the blue chest, which is less important.
So, at level 4, 220 points to get 2 blue chests, 2 purple chests, and 1 god-like chest,
and at level 5, 180 points to get 1 blue chests, 2 purple chests, and 1 god-like chest.
Similar thing happens when you upgrade to level 6, it becomes even better, 190 points to get 2 purple chests and 2 god-like chest, so the quality of chests is further improved, because when you want 2 god-like chests, at level 5 you need 230 points.
If you don’t think about get all chests (and it’s very difficult at level 8 when you need to do 10 skulls), the higher harbor level will always be the better option.