Sorry to hear your birds are feeling unwell.
Could you post a video of the chickens sneezing? To post a video you'll need to upload it to a video platform like Yotube or vimeo, then copy/paste the link here. Could you post a few pictures of their setup? Is there swelling around their faces? Is there any rattled breathing going on? How does the discharge smell?
Sneezing in poultry can be caused by environmental issues such as dusty feed, dusty bedding, allergies, but, a lot of the times, its related to respiratory disease. There are several respiratory diseases that can affect poultry, but we tend to Coryza, MG (mycoplasma gallisepticum , and infectious bronchitis the most. If you'd like to read into these respiratory diseases you can read the links provided below.
MG
https://extension.umd.edu/sites/ext... Mycoplasma gallisepticum (MG) Infecti....pdf
https://www.merckvetmanual.com/poultry/mycoplasmosis/mycoplasma-gallisepticum-infection-in-poultry
Coryza
https://thepoultrysite.com/disease-guide/infectious-coryza
https://www.merckvetmanual.com/poultry/infectious-coryza/overview-of-infectious-coryza-in-chickens
Infectious Bronchitis
https://www.merckvetmanual.com/poultry/infectious-bronchitis/infectious-bronchitis-in-poultry#:~:text=Infectious bronchitis is an acute,be caused by some strains.
(If they do have a respiratory disease) Instead of treating them with a broad-spectrum antibiotic, you can send some swab samples into your state veterinary diagnostic lab, or a national lab like Zoologix so you know exactly what disease you dealing with so you can choose an antibiotic best suited for that disease. But, sadly, with most respiratory disease, antibiotics will only alleviate the symptoms, and during times of stress, symptoms may reappear. That is why some folks on here, depending on the severity, choose to cull their flock, as there will need to be egg/meat withdrawals put in placed during/after antibiotic therapy, not to mention, after some time the antibotic might become useless, so a stronger one will need to be chosen. Links to where you can send swab samples to are provided below.
Testing.
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
https://www.metzerfarms.com/PoultryLabs.cfm
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf