Indeed, there is a possibility to change the server used by the built-in foundation.e.ntfy:
Scroll down to section “Let’s Hack In”:
“As a developer we can access the ntfy UI through the system terminal, for example through an ADB shell (running this command on a USB-connected computer with “adb” tool installed, with ADB USB debugging enabled in the phone’s developer settings):
adb shell am start -n foundation.e.ntfy/io.heckel.ntfy.ui.MainActivity
…"
It’s been a while since I tried this (with /e/OS 3.1 or 3.2), but as far as I remember, this adb command opens the settings for the built-in foundation.e.ntfy. There you can change the default server.