I do not think a closed time loop necessarily turns Venat into an agency-less slave to causality. Armed with foreknowledge, she could at any time, choose to act in a way that would change the future. We know she could have done that, because the entire plot of Shadowbringers involved taking action to change a known future. That she chooses not to is something she has agency over, she goes with her plan in order to give our future a fighting chance.

In that sense, she is choosing to value an uncertain and imperfect future known to have suffering and pain - over the seemingly perfect world of the ancients.

(if anyone are slaves to causality it's Emet-selch and the rest of the ascians. Whose actions were known by Venat, and acted exactly how the WoL told them they would, and had no ability to alter that course. But even then, I think their choices were still their own.)