GSSR advice

I mean you already know what to roll for. If you lack a 5* rider why wouldn't you roll for one?

Here's the thing, i do have merlin and tamamo as well as illya so i'll give you my two sense on caster vs rider.
Waver is by far better then merlin and serves a far different purpose then merlin. Having one doesn't mean you don't need the other. Ideally you want merlin waver tamamo and skadi (when she comes out) to have all your main supports on hand. Problem is np2 tamamo is worthless and while i don't know your opinion on sanzang, she is good but not game-breaking like waver. So odds aren't great for waver but in theory its about as high as you'll ever get for him specifically outside of solo rate ups.
Rider has all kinds of great choices but like caster it to has a bad seed in medb. Drake is a good farmer. quetz is burst buster crit damage and ozy is a bit of support and buster damage but very unique. Medb is just memes.
Overall, waver is the best possible choice to get but it also features the worst possible choice in np2 tamamo which means it's a matter of risk vs rewards. Caster has high risk high rewards with rider/lancer being far safer overall in servants but lower tier servants.

Go for Rider. You already have a Tamamo so going for Rider is less risky. For your question about Waver, Waver is a general support unlike Merlin and Tamamo. That means he'll still works well no matter what happened.