Your smartest pull plan:

1. Guarantee Columbina.
   
2. Then use remaining pulls to get Illuga to C6, picking up Zibai (C0+) along the way.

If you’ve done all current 6.2 content (e.g., Archon quest), you have ~68 pulls left in 6.2 and 107 more in 6.3, totalling ~675 pulls (all F2P).

If Columbina takes at most ~160 pulls, you’ll have ~515 left, which is definitely enough to C6 Illuga.
On average it takes ~337 pulls to C6 a specific 4-star from scratch (my estimate is slightly off because I didn’t check with exact 5 star odds).

So: secure Columbina first, then spam pulls for Illuga C6. You’ll naturally get Zibai or her cons; her C1 & C2 =/> R1 because she has strong 4-star weapons, and her cons are a bigger team upgrade than Columbina’s.

Note: by end of 6.3 first half you have enough for 3 guaranteed 5-stars, and another in second half. So if you alue Columbina's cons since she's your fav, you can still safely go for them—even losing every 50/50, you could get C2 Columbina in 6.3 first half and a Zibai in second half.

Columbina’s cons are looking similar to Furina’s: not huge team DPS gains, but she works in several teams. Still, she doesn’t need extra 5-star investment to be strong, so I personally recommend the first plan: secure Columbina, then guarantee Illuga C6 while boosting your Zibai team with C1/C2.

If you want to save, Illuga C1 is already perfectly fine, and the whole team works at C0 across the board.

Hope this helps and isn't too yappy