There’s no easy way to answer these types of question because of the structural mess D2 is currently in.
In general if you enjoy the game and expect to be onboard for the longer term, then you probably need everything, maybe excluding Forsaken.
If you are just testing the waters you either go free, or buy the season pass that will give tons of materials while you play. Yet this isn’t a good idea, I’ll explain at the end…
So, the general problem is what I described in the whole thread and that has been covered even by general news, there’s no “starting from zero” because the typical progression to the content is not there anymore. You start at power level 750, and that means above ALL legacy content.
This means that all the content part of the ideal progression from zero is already “behind you”. You can do it, and there’s a good amount of stuff to go through, but it’s very, very easy, making the gameplay itself very bland, and so not even a good introduction to why people play Destiny.
At the same time, every tiny thing you do as you step in the game counts as character progress. You could even ignore all that content and you’d still progress just fine, and really fast. “Content” and progression are disconnected. Going from 750 to 900 is very fast even playing very casually. The first softcap is at 900, at that point the star map will give you icons for those activities that give powerful rewards and that make you progress from 900 to 950. Even here, none of the “content” matters, for character progress. From 950 onward progress grinds to an halt and is essentially reserved to dedicated guilds, more or less.
For all this none of the content enters the picture, at all. The only exception being the stuff you wear and use. Some exotic quests for special weapons are indeed tied to Forsaken, Shadowkeep and Season pass. So, character progress is one thing, removed from content. Item progress (or rather type, unique items) is instead partially tied to what you buy.
By default you have plenty of stuff available, but there’s “more” stuff behind the paywalls. (but, even then, much of old stuff is mechanically outdated anyway, if you care about performance)
Forsaken itself is a separate purchase. It was the best expansion and content, but this isn’t valid anymore: lots of content of Forsaken is already integrated in the free version. Then some other stuff, like a lot of the post game and some of the quest flow that made Fosaken a meaty and remarkable expansion, has been gutted entirely. It’s just not there anymore. So, after you remove the stuff that was cut, and the stuff that is part of the free game, what’s left is the shorter campaign and the exotic items still part of that purchase. When you buy Forsaken today you don’t buy the Forsaken that was and many players still recommend. The shorter campaign is still the best campaign, but it’s both made easier and much shortened compared to what it was originally.
Then there’s Shadowkeep and season pass. I don’t know exactly when, but this week the current season pass ends. All the content is gone. IF you buy Shadowkeep AFTER the current season ends, the purchase will include the next season pass, covering the next three months (don’t buy anything until this happens). The pass by itself is $10, very, very scarce of content, but gives you tons of mats that are useful if you plan to play the game instead of just visiting it.
So, you either go free, and won’t miss much. But it means you’re just visiting and aren’t exactly on board for what Destiny wants to be. Or to be up to date you’ll need Shadowkeep, that includes the Season Pass at some point this week. Forsaken being an optional purchase, for that campaign content that is still a pale version of what it was, and whatever exotic quests and content it had.
I’d probably wait a sale to get that stuff.
EDIT: When I mention the Season Pass it’s not the Season Pass you might buy with Shadowkeep, but the single, three months, “Season” that comes with Shadowkeep base version. I think there’s an optional “Season Pass” that includes the whole year.