there was a blogpost that contained hints on how to change the ntfy server for the built-in ntfy but is offline now- here is a snapshot: https://web.archive.org/web/20241209170945/https://wrily.foad.me.uk/going-google-free-with-unifiedpush-in-e-os
and here is a related issue with FOSS warn app which was solved by these hints … well, least from what I understand…
… maybe there´s something in it that you can make use of