It took me all of five minutes to get ntfy.sh (self-hosted) running on one of my cloud VMs and send my first test push to my phone.
Less than an hour later, I've got all my other servers set up to send a simple notification with an authenticated curl POST when they finish booting.
I'm VERY pleased with ntfy.sh so far, and the docs are really impressive and helpful too. My mind is spinning with all the other stuff I'm gonna set up with this!