The point of outbox is that all of the other lists are optional. Set YOUR outbox and everyone you follow has their outbox relays dynamically served to you. If you want to do a blast broadcast, specify a proxy relay, or makesure your DMs go through a specific relay, THAT is when it gets complicated because you are a power user. Those toggles aren't for the average user. Literally selecting default relays works for 99% of people.