It is not as simple as that. You touch upon a controversial issue under EU Reg. 261/04 and something that hos not been clarified by case law so far AFAIK.

Just because there is an earlier flight with another carrier, pax can't just book such ticket on their own and seek reimbursement afterwards if rebooking to that flight is denied and IB is able to offer a reasonable rebooking option on their own flights. When IB is able to offer same day rebooking to BOG, pax is not entitled to insist on getting rebooked OAL just because there is an earlier flight with another airline. That would impose unreasonable rebooking costs on the airline and the pax would gain very little, Most will accept this view, in particular in a situation where the delay was caused entirely outside the control of IB. Had IB been able to offer rebooking to the following day only, I agree with your assesment.

Anyway, it is not clear if Avianca had seats in C/J so any claim for EUR 600 comp. will fail already there.