Hi!
Errmm i got into service,tasks ands stuff by analyzing the services given by UE4.
For example UBTService_RunEQS shows how to tick, how to init and so on.
On BecomeRelevant/onCeaserelevant is mostly like what u want, since they get called whenever the service gets activated/deactivated(this means when the BHT is active on the node or a child).
I use becomerelevane/Cease for example to set blackboardkey observers, so ican react on changeing ot a value.
Hope it helps