It's not correct that Forsaken and Shadowkeep are worthless in terms of loot. All of the loot from the raids in those expansions are still 100% relevant, and some of the world drops from those expansions are being refreshed and made relevant again as well. The Forsaken, Shadowkeep, and Beyond Light campaigns are the main "story" content at the moment, and they're all included in Game Pass, so that would be the place to start.
As for the post-campaign "loop," it's somewhat similar to WoW in that you probably won't be high enough item level to complete the end-game PvE content after you finish the story campaigns. The Director will mark what activities can drop higher level items, so the loop would be completing those activities (a mix of strikes, pvp, gambit, etc.) to gear up for the endgame PvE content, which at this point are raids and grandmaster nightfalls (similar concept to a mythic dungeon in WoW). Those endgame activities drop unique loot. And of course, PvP is infinitely replayable if that's your jam and has unique loot as well.
If you haven't played since before Forsaken, they reintroduced "random rolls" with Year 2/Forsaken. For a lot of the people, the ultimate Destiny endgame is grinding for the 'god roll' of a specific weapon. Each weapon generally has 4 perk slots (2 main ones and 2 minor ones), and each "roll" of the weapon will pull from a pre-determined pool of perks for each of those slots. So if you want 4 specific perks on a random roll weapon, it might take a lot of attempts to get one. Some weapons, including all exotic weapons, are "static" rolls, so you'll only ever need to acquire one.
They've eased up on the FOMO recently because seasonal content sticks around for a full year now, as opposed to just the season (destiny seasons are 3-4 months). They've also gotten better at gradually telling a story over the season, with the current season arguably being the best example of that so far. Each season generally has a unique seasonal activity as well as at least one exotic quest, which is story-related. Each season also introduces at least a dozen weapons or so, so there's new loot to chase every few months.
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