Which GSSR Servant should I go for?

Based on who you have, looks like you're covered. So roll caster or Extra. Or roll for Ozy/Quetz if you don't like Medb. Nero is the only aoe Saber I see. Liz is the only aoe Lancer I see. Both of your archers are 4 star. Maybe roll for Cu Alter for the single buster you're looking for?

And the worst atrocity of all: Bryn's Buster Skill is only Rank three, you monster!

Anyways, hope this helps. You look like you're in a good place, so maybe just roll for whoever looks cool or that you like.

First... I'm a bit jealous of your line-up XD.

Second, if it's a rider you want then the Rider GSSR seems to be a good choice for you. Ridertoki is an excellent ST rider Buyr he can't really make use of Merlin's Buster buff. There's a 50/50 chance you'll get Ozy or Quetz (both Great ST Buster servants) rather than Drake or Medb.

But you may also want to consider the Caster Banner. With both Waver and Tamamo, that's a combined 66.67% chance to get one of them over Sanzang.