Instead of passive XP/SP gain from Training Facility, please provide additional facility that will spawn randomly a "low level" missions.
Since late early game, DDA scales all enemies globaly and there are no more low threat missions that could be handled by untrained (not in par with current DDA difficulty) recruits. Such facility could spawn a difficulty downscaled mission, just for recruits stationed in base and manaully selected for duty (limit of mission cap, with difficulty calculated for selected team), like...
- Scavenging (low resources gain),
- Ambush on patrol,
- Local settlement defense (virtual settlement, not Haven defense so lost mission doesn’t do anything apart from casulties suffered by recruits),
- Nest like experience to clear caverns located near base.
...and so on. Missions that resembles threat handled main Phoenix Point forces, but downscaled to local skirmishes. It would, apart from obvious XP and SP gain, give the feeling that not all threats are cosmic and there are local problems someone has to handle. It will make new soldiers training much more organic, and they will at some point join main forces already battle hardened.
PS. Idea for a facility name is "Local HQ", by BoredEngineer user.