I'd like to be able to query and get a list of all magic items/spells/regular items/etc the way I can with monsters in the Encounter Builder. RIght now, unless I"m missing something, if I want to see all the magic items available across all my sourcebooks, I have to go to each sourcebook individually and look up all the items separately per book. SInce I have the Legendary Bundle and have kept it up to date, I have all the books, so I don't really want to dig through each one individually when building NPCs etc. It works for Monsters in the Encounter Builder, so why not with spells, items, etc.?

Did you try searching items and spells under game rules?

In the menu click Game Rules.

From there you can choose to bring up a class or view spells by all, school or class. You can then click links for Races, Feats, Monsters, Magic Items, Background, Equipment and Vehicles. These links take you to the list of such or a specific Search tool. The search tool can be filtered to specific sources, types and categories and more.

The encounter builder isn't even the best way to search for monsters. The lists in game rules were designed to search and filter (but they can't do everything you would expect them to be able to).