Stop public betas from downloading on iPhone
I don't believe it is an option without an MDM, but if you use an MDM you should be able to Defer Software Updates for a set number of days.

If you are running macOS Server you can use Apple's Profile Manager, if not I believe there are a few free MDM's if you only have a handful of devices.

Alternatively, you could remove the profile that opts you in to betas. That freezes your device and when you're ready to upgrade, then add the profile back and do the deed, repeating the removal as soon as you restart to prevent the next from arriving.