Camera stuff is pretty tricky… you may wish to consider using Cinemachine from the Unity Package Manager.
There’s even a dedicated forum: Unity Engine - Unity Discussions
Otherwise if you want to proceed with the above, here’s some relevant notes:
All about Euler angles, by StarManta:
https://starmanta.gitbooks.io/unitytipsredux/content/second-question.html
Notes on clamping camera rotation and NOT using .eulerAngles because of gimbal lock:
How to instantly see gimbal lock for yourself: