The all add to the game, the only dlc that is a waste of money are the silver bundles or buying things twice.

Forsaken adds raid and dungeon and year 2 exotics access (Dreaming City story is free)
Shadowkeep adds raid and dungeon and year 3 exotics access (Moon campaign is free)
Beyond Light adds raid and year 4 exotics access (Europa campaign and stasis is free)
30th Anniversary pack adds a "premium" dungeon and unlockable cosmetics through Dares of Eternity (the Dares activity itself is free)
Witch Queen, Lightfall, and Final Shape all add a story campaign, raid, exotic missions, and exotic gear access for their respective year.

DLC is almost exclusively PvE related outside of the fact that the most recent expansion is required for the Trials of Osiris end game pvp mode for the sweatiest players.

so, i need to buy all dlcs to progress my gear to do the endgame content?

No, you buy the dlc you want the gear or story for, the best or meta gear is constantly changing and I would recommend buying it all so you have access to everything but even if you only buy the Final Shape you'll have plenty of options.

If you bought dlc up until Beyond Light then you'll want start with Witch Queen, Lightfall and Final Shape. "Premium" dungeons are additional purchases and include unique armor, weapons, exotic weapon, and title seal but you can leave that to a later date either when you've got money to spare or after getting used to the game again.

So, basically the gear in the last dlc isn't necessarily stronger than the old ones? Can I play a relatively smooth game without completing it?

I don't know what you mean by "smooth" but you need to buy the content you want to play, each expansion is a stand alone experience and you don't need Final Shape to enjoy Witch Queen or the other way around, at most they're loosely link through a story with Witch Queen "introducing" the Witness and Lightfall leading up to the Witness entering the Traveler with the Final Shape being us pursuing the Witness into the Traveler.

If you're looking for a totally cohesive story experience you can play all the way through then that's not going to happen, there was seasonal content between each expansion that was cycled out for each new year of seasonal content.

Strictly speaking newer gear is not stronger in terms of stats or power level, all weapons of the same type have the same damage depending on the frame and any weapon can be max power level but because new perks get added for new expansions new weapons tend to be more desirable.

For example Shadowkeep added an auto rifle called Arc Logic that could have a perk combo that reloads and gets a damage buff with each kill with a classic subsistence and rampage roll but Lightfall has a newer gun with newer perks such as Rufus's Fury that can reload depending on how many shots you've landed and gets a damage buff if you maintain sustained fire on an enemy with a rewind rounds and target lock roll.