How about Crawdaunt? I noticed your team doesn't exactly have a fast route towards taking out Ghost-types, (importan because Ghost gym) and Crawdaunt is a Water-Type that also has Dark STAB for this.I will start my Pokemon Shield playthrough in a few days and my planned final team looks like this:
I already did some research and I found out that Milotic will be available very late in the game.
So I will probably have to use another water-type pokemon to beat the story.
Which water-types other than the water/flying type ones (I don't like those) can be recommended?
You can either catch Corphish at South Lake Miloch, (somewhat inconsistent) or by fishing in Galar Mine 2. (15% chance there) You should be able to get Corphish by at least Kabu, potentially earlier.