There is a charge time for each Bijuu Dama, and Chomei has not shown the feats to charge them up so fast that its hard to react, nor has he shown Continuous Bijuu Dama, so every single one he fires needs to be charged, giving him adequate time to enter Sage Mode after using Rashomon to deflect the Bijuu Dama.
Also, due to the size difference between Full Kurama and Chomei, the former's Bijuu Dama will be much bigger and much stronger than the latter's, making Chomei's easier to deal with.
He'd block the first one and enter Sage Mode while the second one is charging, and we've already seen how quickly he enters it, and performed jutsu right off the bat. [
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