GrapheneOS accepts donations and, to my knowledge, they spend it in hiring full time engineers.

They have replacement portions for Play Services already (attestation, an app store, and location), but it'd be interesting if they also offered something for push notifications.

If you have it to give you can spend your budget on a donation and fund the effort directly.

I believe GrapheneOS talked about hosting their own unifiedpush server and baking it into the OS not long ago