A very interesting video about the Thunderbird Project successful donation process and how KDE can improve them by following their step.
A CalDAV server doesn’t do notifications. Its job is to store event definitions, period. Even if it wanted to, it can’t interpret the definitions (because it’s not its job)
No, you’re wrong. Gmail as CalDav server does it, it emails everyone when you setup an event. Baikal also does it but its kind of rudimentary and Radicale has a ticket open for it.
The Google apps are a groupware suit (which happens to have a CalDAV interface, which is incomplete btw because Google likes to keep some features proprietary).
If you’ve read the Radicale ticket you saw that the contributors agree with me and aren’t eager to implement such a feature.
I’m just trying to help you. If you need email notifications you should look at groupware products like NextCloud, not CalDAV servers. But be aware that groupware software often implement some parts of their stack as proprietary or non-standard.