Anniversary Blonde

Asking who to burn, right? Honestly for efficiency sake I'd just burn Illya since you have sanzang even if she's a nuke, but I also wouldn't since she's a five star that you got for that 1% that your luck hit.
I wouldn't burn Chevalier since she (for me, that is) would help you quite a lot with tanking for her and other glass cannon DPS but if you can do, do it burn her since she needs at least NP3 to function really well, so for the other two that's one and just like what you said caster Marie, lancer Eli, and Stheno, burn them if you want.
For the other two it's merely in your choice.
Nitocris is a great farmer, but she's just that honestly and would fall down on gold enemies according to what I heard (pls correct me if I'm wrong)
Medea Lily whom I personally really want is great at healing and would make any berserkers live longer, but that aside she's just that.
Berserklot isn't worth it to burn. He's a crit berserker after all.
But if you're asking my opinion. I'd burn Nitocris (even though I don't want to), Stheno, Marie caster, Chevalier, and Eli lancer, but honestly just burn illya if you have any efficiency and don't really care of burning a five star since it's better to lose one Gold servant than five. or just wait to NP them and burn extra copies lol.

Burn Stheno

5 rare prisms for a mystic code?!? I wouldn't make that exchange lightly. Personally, I don't bother using anniversary blonde, only a few times when it was new. The combat uniform is the only code I use any more. Is that quest still available for newer players? I recall it didn't have a time limit when introduced (thankfully, I was too new to complete it when released).

Burning Ilya is sacrilege. I'll have Ibaraki "handle" you for that.

Just get it later. Rare Prism shop will always be available. You will continue to accumulate useless to you 4-stars as you play.

On one hand if you have Xuanzang she fills he same niche almost as illya albeit using a different playstyle. On the other hand illya is a limited servant. I got her and even though I'll likely never use her, it's hard to just delete something limited. Same thing with Marie caster. I'd burn the non limited ones though if you really wanted to as they may spook you in future rolls anyway

Why do you want Anniversary Blonde? Though nice to have its value is definitively less than the flexibility offered by having another servant, I mean if you want it because you like it thats fine but if you're going for it for gameplay reasons having the extra servants will just be better.

Just a friendly reminder:

Only burn if you have a dupe for now...

Because:
- The servant might get an Interlude and/or a Strengthening Quest (which can give you Quartz after you finish, it depends tho)
- Bond Levels gives Free Quartz (4* gives 2 Quartz from Bond 6 to 9, making 8 Free Quartz total, and 5* gives 3 Quartz from Bond 6 to 9, making 12 Free Quartz total)

So... Yeah, only burn if you have a dupe since you can get free quartz off them. That's why I didn't burn my Stheno yet.