Sorry but this just is not understood, why discovery is such a big issue. Are these devices being reconfigured constantly? If not why do they have to be rediscovered frequently?
Consider a large router for which the secondary discovery done by snmpcollector builds a profile with a thousand resource items. That effects the local database where the profiles are kept, and that's all.
You start out talking about the load by 'monitoring' and then focus on discovery. Discovery is an occasional event and monitoring is on going so it sounds like the need is to restrict 'monitoring' and not 'discovery'.
If a template is setup with only the one QoS you want to monitor then that's all that will be polled during each monitoring cycle, regardless of how many resource items are in the profile tree.