Sounds like a simple Weaponskills and OGCDs (Avatar swings during attack animations during the preview) build your gauge, then your activation drains it to transform.

The emphasis on it "Not being a pet job" from SMN and MCH is kind of funny. That means Dark Knight is viewed as a pet job too. This cements the Avatar animations are just OGCD attacks.

"You decide when to bring in the Avatar" means it's likely a duration based on how much gauge you have. So you'll have some windows of flexibility. You'll do 10 second transformation windows here and there to ensure you have 20 second transformations during maximum buff and uptime segments.