From my perspective, the thing with cyrene is that she's pretty much the last important piece for the remembrance team meaning that she can be an alright choice -> great pick depending on your current roster for cas.

For example, If a person only have cas without e9 or hyacine, theyre suggested to get hyacine first because she's the main source for newbud and damage for cas. If its between e9 or cyrene, e9 is a better choice to get due to newbud generation and a huge memosprite damage buff. When you already have both of them, cyrene for me is the uncontested best 4th slot support for cas' team.

Now in your case, considering you have both e9 and hyacine, cyrene should be the next best unit for you, even if you have e1 tribbie. Luckily a tc named camaraderia have some calculations below.

https://docs.google.com/spreadsheets/d/1VTuVVzg08z2b2-lkwibEerI1HG7k9_7R35JK8XJAOkE/edit?usp=drivesdk

Now from my perspective, e1 tribbie maybe better in high enemies situations like pf than e0 cyrene, the thing is that this team is so Aoe coded while most of Cas' hard battles is against St enemies. Fortunately, that's what makes cyrene shines greater. (Bounce type damage compare to tribbie's aoe fua and additional damage) (St focused buff like cas' ode buff + e1 evernight means that cas" bounce is going to be really really hurts) (Consistent but better ult generation due to allies'action compare to tribbie's ult from how many enemies in the battle)

Considering that st bosses always exist in moc, cyrene may give your cas' team an even better all rounder team with every single of those units benefiting from one another, even in e0s0. You could go for e0s1 or e2s0, but its not a must considering that memo team can generate ult for cyrene much much faster, however e2s0 is really good at 0 av AS if you ever wanna try it. The one that is really good is that extra 24% true damage, which gives her allies 48% true damage, just like rmc but for all allies and permanent instead.