seems like you have it back to front
outbox means your follows tell you where to look, that's a manual path
gossiping proxy relays are gonna have to use some kind of data set to make decisions about where to look, and use your follow list to dictate what you want to look for, which means it is gonna require approximations, guesses and brute force, expensive, and slow scanning traffic
then, separate from this, is where you look by default, your "app relay" set
just look at how #nostrudel implements it
it gives you the best of both worlds, like a hybrid stick shift
and if i was to choose what way i'd prefer, i'd go with the stick shift, but look at how it's implemented on a motorcycle: up and down...
so it's a bit of a mess of analogies to talk about but let's just say that i think "outbox model" should be renamed "rendezvous" because that's what it's about .... meet me here, i'll meet you there, depending on who is initiating
as a relay dev, my biggest problem with the views you are expressing is that as a user, it gives me ZERO control over where my client looks for data, whereas with NIP-65 i define where i publish, where i search, and everything outside that, ok, let's go goblin mode, but if everyone can see this data most of the time goblin mode will be unnecessary