I'd love to pick up Milotic as well, but outside of Floatzel idk what I'd cut, plus I'm *horrid* at making poffins and don't have Switch online to evolve a Feebas the other way. When I first picked up Golduck as a Psyduck I never meant to keep it on for long so *maybe* I'd cut it, but it still wouldn't solve the problem of I can't get one, so its probably not gonna happen unfortunately.
I don't see myself dropping Gastrodon or Gyarodos. While I probably don't actually *need* it, I value Gastrodon's electric immunity a bit too much to get rid of it, at least not before Volkner. And Gyarodos at the moment is probably my hardest hitting team member so I don't want to lose all that damage. For me there's not really any other water mons that'd be worth losing either of those things for, maybe Palkia, but I'm on Briallant Diamond so it shouldn't be available for me without trading and again no Switch online.
I definitely agree Storm Drain and Surf is a fun combo in doubles. I've already got one Storm Drain user in Gastrodon but I keep forgetting to set up Surf on some of my other mons so I haven't gotten to start using it with this team, I should definitely do that next time I get on.