Different coastlines have different ruggedness.  They are not all the same.

So assign [different fractal dimensions to different segments](https://en.wikipedia.org/wiki/List_of_fractals_by_Hausdorff_dimension#Random_and_natural_fractals) as you generate it.  

E.g. Great Britain is 1.25 but Ireland is 1.22 and varies a bit around the island.  Norway most famously is 1.52.

This should work with the geology if you’re doing that level of detail — different kinds of rocks wear differently.

Others commented on your lack of disclosing your projection.  Drawing the coastlines on a plane (even if the overall shape is projected first) will not look right!

As for the sea-sized inlet you asked a about, that could very well be a rift.  Expect the surrounding geography to be consistent with this.  And the larger point is that particular features don’t occur in isolation, but they interact.