Ducked Delay would probably be the easiest starting point. You’ll be limited to a 3 second crossfade, if that works for you. I see any downstream delay as being a sync’ed phrase extension to the Looper. It may be a matter of simply HotSwitch-ing the Ducked Delay Mix between 0 & 100, for bringing the effect in & out (assuming Looper -> Ducked Delay in Series).
The Slow Gear attack … yes, that will require some other tactics or algorithms. But if you’re mainly interested in crossfaded drones, I can think of quite a few other algorithms that might fit the bill, with less of a limitation on the overall durations.
