I mean, I can’t. Since late early game there are no more low level missions. DDA scales globaly, so recruits will face threat they’re not supposed to handle. 
I would go with Local HQ to handle maximum of soldier mission cap, and occasionally spawn a mission just for them with downscaled difficulty to be doable by the recruits in Local HQ. Ambush, Scavenging, Settlement defense (virtual, not Haven defense so lose doesn’t do anything apart from casulties suffered by your recruits), maybe even Nest like experience to clear caverns located near base and so on.
It would give also the feeling that not every threat is crucial or lethal, and there are still some skirmishes around that have to be handled by someone.