I'll tell you what OK State Veterinary School did when I had my barn full of strangles. I bought a horse at a private sale, brought her home, 30 days QT just like you did. Nothing. Turned her out, and BAM! lumps under the chin and snotty yellow green gunk out of her nose. I knew it was strangles before I ever pulled her out of the pasture. I had 8 horses in the big barn down with it. Two mares had already been moved to the foaling barn, so we locked down the big barn and no one who went in there crossed the line to the foaling barn. We installed foot baths in front of all entrances and exits to both barns. I posted on the gate that the farm was on QT and no one was to enter past the gate. Employees left their vehicles outside the gate. Employees stepped in the foot bath at the gate before coming in further. At the feed barn, they put on DuPont Tychem suits before working in the barns or with horses.
The stallion developed b*st*rd (internal) strangles, so was in isolation QT in the ICU at OSU Vet Hospital for 30 days, with a trach in his throat. Touch and go for a while.
What we did for the cultures was every horse twice and the mare who brought it on the property and the stallion 3X (since he was the sickest). Once everyone checked clear, the QT was lifted and we resumed our normal schedule of training, trail rides and shows. During QT (which since everyone came down sick on their own schedule, ended up being ALL summer and into the fall) there were no horses in and no horses out of the property. I took all deliveries at the gate, no UPS or FedEx or USPS allowed past the gate.
Footgear worn into the main barn was left on the property and disposed of once everyone was clear. A sign in & out log was kept at the gate for every person who came on the property. Anyone who came in past the gate was advised to go home and shower and change clothes after leaving, before they went anywhere else and especially before they went to another farm.
Thanks to the strict management of the QT, the pregnant mares were not infected, nor were their new born foals once they delivered. Expensive? OMG, yes, it was horrendous. But it kept everything contained to this property and not spread all over the state.
The cultures are expensive but worth every dime. If you have a chronic carrier, you want to know it because any kind of stress can cause them to become active shedders and re-infect your herd or infect other horses (say on a trail ride). If you're able to contain it to just 2 horses, then I would do whatever I had to, to give the gold standard of care to both horses and insure you don't have a chronic carrier. It will pay off in the long run.
Now, when I bring in a new horse, it's 60 days QT, no exceptions.