So I’m going to do a run through and collect all of that good stuff, do I need to be on legendary for any of it?
No you can do it on whatever difficulty you want.
Nope. You do have to have 20 characters though.
IGN has a guide with an interactive map that is very useful. If you sign in, then you can check off items you collect. It helps you keep track of your progress and hunt for the things you’ve missed.
It sounds like you’ve played the game already, so I think you should be aware that certain areas associated with the linear missions are inaccessible after you’ve completed them and also you can’t access the open world after “The Sequence” mission.
you can do then better on easy difficulty for the skulls since there is 1 that give’s your trouble if you do then on normal or higher since you cant kill any sentinals in that missions since you most keep then alive and thats the only trouble you get if you get then on any other difficulty.
There doesn’t exist any requirements in terms of playing the game at a specific difficulty to unlock any collectibles. You can collect them on any difficulty that you wish like Easy, Normal, Heroic or Legendary.
When it comes to guides, I would recommend Halo Completionist’s collectible guide for Halo Infinite.
For things like the banished propaganda towers, I found IGNs interactive world map really helpful since I could mark everything off that I had already previously collected.>