Too much traffic for the clients (apps). And batteries draining. Could be doable though on a wifi and a PC.

My tree of relays architecture could be relevant for achieving both decentralization and low traffic:

nostr:naddr1qvzqqqr4gupzp8lvwt2hnw42wu40nec7vw949ys4wgdvums0svs8yhktl8mhlpd3qqxnzdejxqmnqde5xqeryvej5s385z

It doesn't use outbox model but still it is something. In this model cilents would write to leafs and read from stems to both achieve decentralization and low traffic.