I’ve been after this for a year.
The best way I’ve found to reduce mobile data is to run a proxy like @ben ‘s  https://github.com/bndw/nostr-relay-proxy/blob/main/README.md 

 I'd be very curious to hear your thoughts on this (related, possibly identical) topic here: https://primal.net/e/note1nrds8k9cdfeeem6kg58j6tcl00xyaa8z336vjh2h7wz546hj7a9svvcapj