Just make a 3 man minimum party for Ambuscade. The problem is stemming from the ridiculous number of soloists slowly killing the lower difficult modes, just have them team up and call out a few trusts and they could easily kill the same difficulty while getting more gallantry at a faster rate. There is no downside since it would still be a normal run, no need for dedicated support jobs as they aren't doing D/VD. Or better yet impose a 60 min lockout if someone enters ambuscade solo.
It's hordes of soloists that are killing ambuscade because they refuse to invite one another to curb stomp the lower difficulties together.