I'm also at rank 17 with a very similar deck at the moment, so im not sure if I am the right one to give you advice But here are some ideas:
I would definitely add Ciaran - you need at least one lock against Skellige e.g., and he has the movement part which is perfect with your Marksmen.
You could also add Ida Emean the additional Clear Sky is good against Eredin Decks, they often run 3x Frost + 1 x White Frost.
I would throw Royal Decree out and add a Golden Weather cards. That can be very strong if you combine it with MH. First round you play weather with Ida to force out their clear sky. In the 3 round you start with Golden Weather, force them to put all their cards in a row and kill them with MH (and lacerate if you want to keep it). If he gets what you are doing you can still use Zoltan or other movement cards to force them on one row.
Finally I am not sure about the silver spy: You can add him, for he is really good to draw out another weather for the last round.
Thanks a lot for the advices. Putting Ciaran and MH sounds nice, will have to take the Witchers out so I will have another spot for silver. Which one would you with? I was thinking about Decoy maybe, but not really sure. Regarding golds, what do you think about Aglaïs? Against Skellige, she can pick Restote which is a huge move
Depends on which decks you are the most challenging for you. If you have problems with weather (which I had at some point, mainly against Eredin) its good to put Ida in for Clear Skies (or 15 point swing if you dont have weather problems). Roach could also help a lot to gain tempo (which you of course lose a bit when you take the witchers out).
For me it is mainly try and error, putting some silvers in, keep them as long as they work, otherwise switch them for something else.
For the golds: I think Aglais can be nice, I actually dont have the card, but I think she can be really valuable in certain match ups.
I found myself also struggling with decks that have big finishers in round 3 (like pirate captains, spellaetel) so perhaps Gerald Igni or Borkh could be a wise choice
But again that mainly depends on what you are struggling with