If that's the case, then I wouldn't use medication (Ivermectin) to treat for SLM.
You will have to coat legs with an oil. SLM live under the scales of the feet and legs, so the oils/ointments will smother them. You need to repeat treatment at least once a week.
Your best bet with that many birds would be to put oil in a bucket, then take one bird at time and dip the legs into the oil.
While SLM do mainly live on the birds, they can migrate or you might find them in housing since you have so many birds, it seems that they have spread. It would be a good idea to clean out housing and treat roosting areas, nesting boxes, etc. with a Permethrin based poultry spray.