I honestly think you are approaching it from the wrong angle. Yes bank robs are key activities that engage the most crim v cop activity, but at it's current state it doesn't need changing. Cop activity tends to fluctuate - Don't let the current state fool you, there is no need for buffs.
What you should be looking at is the current state of turf wars. Turf wars, Imo, are even more crucial for engagement than bank robs. No clue why it turned out death after the last changes where it was made gang v gang, but everyone used to login to participate in turfs.
It might sound selfish coming from me as a cop, but I don't think it would be a bad idea to involve squads in the turf war script.
Idea being that squads would have to gather X amount before being able to start - Once they reach required amount (discussable) they would be able to trigger the neutralization process of turfs in 1 city. Neutralized zones would be blank & owned by no one.
- Arresting would be a no-go
- Squads would only be able to neutralize 1 city followed by a cooldown.
This would do turf wars good as squad members would no longer kill turf wars by arresting everyone but it also doesn't effect balance as its limited to 1 city.
Though I see how this would easily be cockblocked by cops that would rather arrest than participate in the activity.
For the reset time on banks - Failed bank robs (not 8/8) should be resetting faster than those where the bank robs was succeeded (8/8).