Here is what I think I'm building.
Server maintains 24/7 persistent websocket connections to a set of relays with two subscriptions
1. For all kind 30311
2. For kind 3 where author is one of a set of n'000 pubkeys that have requested push.
Each time a 30311 comes in, compare the 30311 host to the kind 3 lists, and if the host is present in a user's list, push the user.
But I wonder
Isn't that quite spammy and taxing to maintain 2* 24/7 subs to each of a set of public relays?
Or is it no drama at all?