Predator proofing and serious respiratory disease or Mareks still stresses me out to this day, well over 10 years into keeping chickens.
You have so much time, love and money invested and that can all be wiped in one fowl (

) swoop. For safety, What puts me at ease is tinkering with the coop. Adding more here and there, reassuring that they are safe.
For coop construction, the best advice I can give is AVOID CHICKEN WIRE. Whoever the bozo is who named it chicken wire should be flicked in the nose. It will do zilch against a motivated predator. Use Hardware cloth on openings, around the outside of the coop and anywhere else that makes you feel good about putting it there.
I also suggest avoiding automatic coop doors. They are a fad, but Anything that causes complacency in chicken keeping is a bad deal. Vigilance is key.
For disease concerns, just practice good husbandry and don't add new chickens to your flock without fully vetting and quarantining them. If you handle your chickens, wash your hands before doing so. If someone goes grocery shopping after touching their chickens who have MG, and you touch their same cart, it can pass to your flock just like that!
Pro tip - there are thousands of hand sanitizer stations out there that are no longer needed after the excess production during Covid. Our second hand store here ended up with 5 of them and I snagged a few for around the animals. Thou shall not pass unless you clean your hands to touch poopy farm animals

just start asking the places you frequent if they have an extra or an abundance that you can purchase.
Anyway, a certain level of stress is expected, but the happiness and joy they bring is unmatched.