On a new game, I sneak up there in Caelid as soon as I can swing a bleed weapon to gank that sleeping dragon and get the higher level smithing stones near it. Once that is done, I finish all of the Lake Region up to the lift before I ever come back. Most of it, aside from a couple of the outdoor bosses, is quite doable before Atlus. This is especially true if you want to build around specific weapons found their like the GUTS, Moonveil, Naginata, ect. Really the best indicator of progression is what stones drop in the dungeons found there, which places Caelid before Atlus.I think Caelid is singularly responsible for a ton of the "Elden Ring is too hard" articles/posts/memes and whatnot.
Where should Caelid fall in the progression? I think after Altus Plateau but before Mountaintop. So probably like... Mt. Gelmir / Volcano Manor / Imperial Capitol tier? Maybe before those?
Seconding that Southern Limgrave is the key and I didn't start making progression in this game until I went to that area. I think that should be the goal before Margit is. Like Margit should be one of the last things done in Limgrave. A lot of people wouldn't be beating their heads on that fight that way.
But yeah, I bet a ton wander into Caelid well before they're supposed to and think it's the normal game progression and get rolled. The single biggest thing I'd tell new players to steer clear of (well besides Caelid itself) is that trapped chest that puts you in a nightmare-dungeon in Caelid. That place took a couple hours of my life trying to get out and I could see a lot of people just noping out and missing out on the game.
My standard progression goes like this when I start over (bear in mind I am ruching to a +10 weapon asap) a new game:
Fuck around Limgrave long enough to be able to weild one of the three bleed weapons found there (Reduvia, Flail, or Utichunga Katana) easily.
Go kill sleepy dragon in Caelid (nailing boat boss 1 along the way) and level up to the intended weapon build I am aiming for. If sorcery, I do the early rock toss/grav staff grab around here too.
Clear Heights fortress and grab snap emote from Blaid.
Go to the south island area and clear it all, finishing with the fortress (esp if I am going unga bunga).
Clean out Stormveil, which should be simple at this point.
Clear out the lake, knocking out the Manor before the castle in order to get Rani quest and Poop Man Summon questlines started.
Go back to Caelid to get second amulate half from the rape fort near sleepy dragon, then do first underground river area (this can be skipped depending on build)
Knock out Radahn battle quest then go do underground city for Rani quest.
Do Selia to get Mellicent quest chain going.
Clear overworld crap in Atlus for whatever you need and bells. Knock out castle as early as possible for robot arm for Millicent quest.
Do White Mask quest to get access to Mogh's area, then suicide grab Ancient Stone and bird farm if needed.
Clear as much of Volcano Manor as possible short of Rykard (if you want the assasin quests) to get all the stones needed to make a +10 weapon, plus getting certain seals depending on build.
Clear out outer moat area of Capitol and any remaining dungeons of interest in Atlus, Mountains, or Caelid. Then broom out the entire capital, grabbing the sewer camp asap.
Once all bosses are cleaned out, broom out the Sewers all the way down, then clear anything desired from Mt Gelnir that you might have ignored.
Go rape Mogh now if your build works for it, then proceed to Giants area clearing it all and getting the secret area cleared out asap (lots of good shit here).
Beat Firegiant but don't burn the tree (need to decide on Frenzy and grab any missed loose ends in the capitol) until you have cleared the entire Haligtree from top to bottom.
Die 40 times to Melina before pussing out and summoning another player.
Burn tree and clear all of Faram Azul, doing Placidex last. Then go back to ashen city to grab a few things and finish the game.