Relay notifications from one public server to a private one

[schklom]

💡 Idea
I have a self-hosted ntfy instance at my_ntfy_url.domain.
Problem: my URL is unique, so giving it out to the Internet somewhat identifies me.
What I tried and failed: On ntfy.sh, if I subscribe to my_ntfy_url.domain/topic1, my browser (and phone app) connects to my_ntfy_url.domain.
Solution: On my_ntfy_url.domain/topic1, subscribing to ntfy.sh/topic2 should make the notifications received by ntfy.sh/topic2 go to my_ntfy_url.domain/topic1 and then to my device. Similar to https://docs.ntfy.sh/config/#ios-instant-notifications, but also for other client devices, so topic2 would be the sha256 of my_ntfy_url.domain/topic1.

The easy alternative is to connect my devices to both servers, but on a phone this requires battery to maintain 2 Websocket connections.
This feature would let my phone maintain only 1 connection (to my_ntfy_url.domain) instead of 2 (ntfy.sh + my_ntfy_url.domain), and the notification sender would only know about ntfy.sh instead of my_ntfy_url.domain.

If usable with UnifiedPush, this would solve the privacy issue from simplex-chat/simplex-chat#1081 where the self-hosted ntfy server can be used to uniquely identify its owner, because the other party would only post to ntfy.sh/topic2. Ideally, this feature can be used with and without UnifiedPush.

I understand this would add a connection from the server to ntfy.sh, but at the same time this would allow to avoid connections from devices because they would only need to keep a connection to my_ntfy_url.domain instead of my_ntfy_url.domain + ntfy.sh.
In my situation, i have a few phones that would stop connecting to my_ntfy_url.domain + ntfy.sh, and only connect to my_ntfy_url.domain.
So the load on ntfy.sh might decrease a little.

To relay with UnifiedPush, maybe a toggle in the mobile app settings could say "Relay new UnifiedPush subscriptions via the following server: (default ntfy.sh)".

💻 Target components

ntfy server (functionality to relay notifications)