Ishtar or Enkidu

I am not sure..You are quite lacking of SSR servant so both are good.

for Gameplay, Ishtar is just slightly above Enkidu. Enkidu need his 2nd skill buffes. But this question is not related which servant is stronger. You see, all Archers are quite good even 3 star. But Lancers...

As you mentioned, Cu is top-tier for Stall, but beside of that, almost nothing. Li Shuwen is great for that 1 turn killing, but beside of that 1 turn, almost nothing. Romulus? No comment due to frustrating by 40% salt. Vlad's overall utility is better than the rest 3. Enkidu is jack of all trade lancer, pairing with Vlad can make your Lancer team have good durability as well as damage.

For Archer. If you can make good team for Kuro/Robin, they can do decent damage. Off course, it is also recommended to roll Rin for a AOE function. Also she is limited.
And rolling for her in Christmas will bring you CE that make you dig deeply in that lottery event.

So I dont know. I slightly favor for Rin.

I would go for Ishtar. After Enkidu comes out EVERY time you use SQ their will be a small chance for Enkidu to drop where as Ishtar will show up at rarely from banners.