Advice on GSSR

well as a f2p who is going to roll for the gsrr for the first time too xD i would say:
either roll for assasin or rider
riders :ozy , quetz , drake and medb 3 out of the 4 are good
assasins : hope you get jack
as you said the available sabers aren't that good and your archer class is well covered i would say and except tesla the others aren't that good so ..

in my case i will roll for a caster since i have no 5star caster and all of them are good

ps: cleo is really good but jack is just an op star generator but i would stay cleo is more than enought
you don't really need cu alter since you have 2 of the best berserks available
i hope i could help :D

Just food for thought,
While none are game breaking, the sabers are all new for you and work well with Merlin. Less Fragile than Raikou despite how hard she can hit.
2 of the 4 Riders are top tier if you exclude Merlin and Waver. As someone with NP1 Merlin, Ozy, Drake and np5 Kintoki rider, Can confirm even with Ozy skills mostly maxed, Kintoki NP hits harder than Ozy's. As far as I remember it hit at least twice as hard. Merlin can't negate that difference, although Ozy is a better team player and has better synergy with Merlin.
Both Caster and berserker banner- sounds like a 66.66% chance of disappointment. Is it worth the risk when you can definitely get something new, at worst, a servant who is good, and useful?

Cu alter is good, but nightingale and vlad probably aren't the best for you right now. Waver isn't as high a priority for you as others since you have Merlin.

While Santa alter is fine as a rider, three out of the four riders in that banner perform better than her: Ozy is good at both offence and support with a very rare buff in his third skill that nothing else in the game can give you, Quetz is more consistent with her damage and Drake, while probably not having as much np damage as np5 Santa, can easily spam her Np and can also net you lots of fp when her skills are levelled.

I would've actually recommended assassin + extra for you since both servants on it are good (or, Jeanne becomes good later on), especially since you don't have the sq for king Hassan. Cleo + Merlin is good, but fundamentally different from Jack. I get the sense that you guage performance by damage, in which case, I don't think so (don't quote me on that one though, I didn't run any numbers).

That said, if you don't want to roll assassin banner, I would recommend rider, then caster, then berserker, in that order.

Your list sort of screams Rider (in my opinion)
Saber wouldn't be a bad option either because they all work well with Merlin (Even Ecskcalibur Pendragon)
So in my opinion, go for Rider and if you don't like it, go Saber because basically all the other classes are filled in with decent to great servants.