remember, if they are vaccinated they are (usually) carriers. keeping illness out of a flock is nearly impossible, for a home flock its usually better to build an immunity and do the vaccinations. but once you start, you cant stop - every new chicken has to be vaccinated too.My original plan was in fact to cull the flock , clean/ disinfect the coop and replenish flock with the exact type of birds that i want to serve my needs from a reputable breeder that are already vaccinated etc. i just need to find a hatchery near me in Caswell County NC
i do think you have the right plan for now, your flock has suffered enough loss and illness it may be best to start over. eventually some sort of illness will come into your flock again, just be prepared- have a quarantine area for new chickens and sick chickens. deal with issues as they come along, if your eating your eggs you wont want a flock that is always getting antibiotics.