You don't need all that. When I 1st ran the Realms I only had the 1994 boxed set. You can easily run with the Grey Box, FR adventures, FRCS or the 4E book.
Yeah, but the problem is, none of those books are entirely compatible with each other.
First there's 1e. You start with FR Campaign Setting greybox from '87.
Then 2e comes about, and the 2e FR Campaign Setting and FR Adventures. And the Time of Troubles. A bunch of gods change, a bunch of lore changes, magic changes, races change, and a few lines on the map change.
Then 3e comes out, with the FRCS white book. There's no real cataclysm, they still change a bunch of gods, a bunch of lore, magic, races, and a few lines on the map.
Then 4e comes out, with the FR Campaign Guide and Player's Guide. And the Spellplague. A bunch of gods change, a bunch of lore changes, magic changes, races change, and a few lines on the map change.
Then 5e comes out, with Tales from the Sword Coast. And the Sundering. A bunch of gods change, a bunch of lore changes, magic changes, races change, and a few lines on the map change.
What I dislike about Forgotten Realms is that every edition there's a cataclysm or upheaval to canonically explain with lore the change in mechanics, and even when there isn't, they still overhaul the entire campaign setting. It feels like the rug gets pulled out from under you constantly. Deities don't feel eternal. They feel like soap opera characters. Lore doesn't feel consistent. It feels designed by committee. The physical laws of the world seemingly change with the wind. They reinvent the campaign setting every time the rulebooks change. It makes it feel like the game rules dictate the events of the campaign setting, rather than the campaign setting being a fictional world that players and DMs get to explore using whatever game rules they like
and making their own stories with it. It feels like the tail is wagging the dog. It's WotC saying, "We changed the game rules, so you must update your campaign setting." It feels like Forgotten Realms only exists to sell the current version of the rules. No matter how true that is from a business perspective, I shouldn't notice that that's the case when I open a FR setting book.
Let's say I make a Middle Earth campaign setting for my role playing game set during the Third Age. Then I change the mechanical rules of my game for a second edition. I include a cataclysmic event called the "Dragonbirth" and now there's dragons all over Middle Earth. Rivendell has been burned to the ground, and much of Mirkwood and Fanghorn was destroyed. Would you be interested in running
every future campaign in this Middle Earth, knowing that all content produced in the future from adventures to new revisions of the campaign setting would never go back to the time of Mordor and the Ring of the books?
Or I do the same think in Star Trek. Then in my revision for second edition, I destroy Vulcan before the original series even begins. Even if my new lore is fantastic, you might see how longtime fans wouldn't be interested in an entirely new timeline. Similarly, if I completely change Klingon appearance I need a pretty good excuse for the changes.
Now, no, there's nothing at all stopping me from running new content in the old campaign setting... except when that new content includes new revisions. If in my chosen FR setting Bane is dead, I'm going to be a little irritated when Bane is the main antagonist in your new adventure. Worse, what happens when DM knows 1e FR, player 1 knows 2e FR, player 2 knows 3e FR, etc. It's a vipers nest of mixed continuity and nobody is looking at the campaign world the same. We're all sitting at the same game table in the same campaign, but somehow we're not actually playing in the same campaign setting anymore.
Bottom line is that consistency, not variety, is key to forming a campaign setting with memorable lore, and it's memorable lore that makes a setting worth playing.