You don't really need specific out of combat tactics for this, just some creativity provided they keep the tactics system as it was in DA2.
One way to do it is with skip tactics.
Enemy: At least one is alive -> Skip Tactics
Self: Any - > Deactive x
You put this in the last 2 slots in your tactics, and when the fight ends, the sustainables deactivate. This would work in DA2 because you were in combat a few seconds after killing the last enemy. If that wasn't the case you could instead make it "3 enemies alive", so it would deactive when there were only 2 enemies left alive, which is minor. It would look like this
Enemy: At least three are alive -> Skip Tactics
Enemy: Rank is Elite or Higher -> Skip Tactics
Self: Any - > Deactive x
Then you can use the "at least one alive" at the first slot to activate the sustainables you want when entering combat, or you can tie it to Mana percentage. Like for example,
Self: Mana < 50% -> Activate Panacea
This way you start at full mana and when you get low enough, healing aura will active and cost nothing. Also you can tie it to the health of a party member.
Self: Mana < 50% -> Use condition for next tactic
Ally: Health < 75% -> Activate Panacea
This way, even if you are low on mana, unless someone takes a lot of damage, it won't trigger.