Yup 500ft/min is the minimum expected in climb/descent. If you can't do that flight crew should advise us, but we are constantly watching your rate ourselves anyways.
If it's a slow descent, and we need you under traffic we'll tell you about the traffic and ask for a specifc rate of descent to ensure seperation. For slow climbs, normally we wouldn't give you a clearance to a level above any traffic anyway, so that there is always positive seperation in place. Often for slow climbers leaving our FIR, we will call up the next controller and ask if they will take them in the climb.
For level capped waypoints, often I will say "When ready, descend FL330, cross waypoint xxx level". If there was traffic in the way, it would be a case of a vector to ensure lateral seperation, an initial descent, then when clear of the traffic, own nav back to the point and then the when ready clearance.
Cheers