Hi and welcome
I am so sorry to hear of your losses.
I agree that more than one death needs to be investigated and a necropsy is the way to go. My guess (and that is all it can be) going on the age and breed is possibly Marek's disease (especially since you are having the deaths a week or more apart) or coccidiosis. These are the two most common killers of juvenile birds. Corid is easy to get hold of and administer to treat for coccidiosis and will not do any real harm if it isn't the problem. Unfortunately there is no recognised treatment for Marek's but supportive care with a good quality poultry vitamin supplement and perhaps a probiotic to support their immune and digestive tract can help and remove adolescent cockerels from the flock to lower the stress levels and keep them as calm and happy and nutritionally well fed as possible.