What lore do you ignore?

[Cleretic]

Venat is no mastermind of fates; she simply considers the future that you describe to her and accepts to take her part in it. If anything, she is the one who has lost agency of her own future and can now only play the part that has been told to her.

...or perhaps she did struggle against her dictated fate, and she was able to act differently and split off a timeline that we will never see but the ancients are saved ultimately due to the warning we gave to Venat – but that can't happen without the original timeline still happening as well.

My read is that she actually didn't do either of these. She doesn't accept her place in The Time Loop; the scene right after Ktisis basically has her saying that she'll look for other options if at all possible. It just seems like those options weren't really viable. ANd yet, we know she was pursuing them even before the Sundering, because of the Anamnesis Anyder discussion.

But even when we have completed the time loop, we know she's not sticking to it: remember that when we meet her in the Aetherial Sea, we learn that her plan is several layers more complicated, reliant on both events that happened afterwards that couldn't have been part of what we told her (specifically, tagging Meteion) and multiple factors we didn't even know were on the table after we left her alone (for example, the Ragnarok).

If she were simply following her assigned role with no agency, everyone would've been screwed: it was her rejecting that 'future history' being told to her and deciding to be more clever that anyone had a chance of survival anyway.

All this assumption about the time loop is predicated on the assumption that our intel was a perfect blueprint anyway, and the entirety of our eventual success was predicated on the fact it had massive holes: even assuming we were as exhaustive as possible, we had a very limited perspective, and gave her a history with a good ten thousand years of '???'. If your information on the time loop that you're allegedly in is so limited, how can you be sure that any deviation you even try to make wasn't just an unrealized part of the loop? Why can't you just try some stuff?

We explained Hydaelyn's Plan B to her, and then found out about the Plan A much later. How do we know we haven't been surrounded by the failed remnants of Plans C through W this whole time?