odyseey started the bloat trend
Not true at all, it started as far back as AC2 already, though AC4 is where they went all in with littering the map with collectibles.
AC2 also started the having a ton of side quests, however you could still ignore pretty much all of them.
AC4 is also where this kinda changed because if you didn't do side activities, you could find yourself underleveled for the story missions or random ship encounters that could blow you to bits in 3 shots; however it was still technically doable, just super hard.
AC: origins is the true start of it all, where you have a ton of uninteresting repetitive side missions that are
required to do.
Required because of the new RPG esque leveling system, being underleveled now suddenly means you can get one shot by an off-screen archer...this game started the design of having to seek out the other activities in order to progress the story, and if you didn't, you'd run into a wall of one-shotting enemies.
AC odyssey makes this slightly less bad, but also worse in other ways.
Again tons of repetitive side missions, but with the way stats and loadout works, you can play with the gear in ways that allow you to tackle encounters even if you are underleveled; the combat system also has no stamina, so you really can't ever lose an encounter since you can just dodge at any moment to slow down time, plus all the godlike combat abilities kassandra gets, means you can generally brute force your way even when underleveled.
The worst thing about the game is the excessive loot-puke like it's a borderlands game.
AC valhalla makes changes to get rid of the nonsense origins and odyssey introduced.
For one there are no longer any side quests, yes the map is still littered with collectibles as introduced in AC4, but you can ignore literally all of them.
The RPG esque leveling system is still there, but with one major change: once you hit level 8 (which is like the first 3 hours or so of the game), you can start assassinating literally anyone, regardless of how much higher their level is; origins and odyssey didn't have guaranteed assassinations, your assassin damage was determined by your stats (far after the games' final updates they added accessibility options, which allow turning on guaranteed assassinations, but since it's an accessibility option and not an in-game mechanic, it's essentially a cheat), making it incredibly painful to do full stealth if you weren't leveled up "properly".
They also brought back many stealth options from past games, like blending and social stealth, so you'd actually make use of the assassinations, there's even ways again to reach targets unseen for a special assassination, and aside from a handful of story based missions, you can play the entire game in stealth.
There's also no loot-puke any more, you can just wear your starter gear throughout the entire game and just upgrade it when needed.
They also nerfed the bird, because it slowed down the game considerably; in odyssey you'd reach an area with enemies, whip out the bird, scan and mark all the enemies and then continue. The bird can no longer mark enemies, so now it's only used to get a quick overview of where the objective might be, instead of a super powerful wall hack.
It's definitely a big game though, but in a way that's different from the previous two, there's more "real content" in it because every quest is unique, there's no repetitive "go kill those 10 bandits" mmo shit like in origins; it's a giant viking "epic" that's more like the main story of ac2, brotherhood and revelations combined, and it's a huge plus you're never level gated like in the previous games.