Last time this happened to me, there was neither Babylon nor Gaul. And even if Gaul is in the game: Turn 30, really? I don't even play on deity (mostly emperor, sometimes immortal) and the barbs prevent me from having a chance of winning in about 9 out of 10 games. As far as I remember in Civ 5 you could choose between "normal" and "raging" barbarians. In Civ6 barbs are always raging. And if you turn them off, the AI will destroy you even earlier.
Yes it can happen on turn 30 with Gaul in the game, because as soon as they plop 3 mines down, they have the full tech for Men at Arms (Apprenticeship).
You should get the mod "April AI Science changes reverted", which should help a bunch on counteracting the AI getting very high early science.
Either way, make sure you don't let the barb scouts spot you and you should be golden, even on Deity.
And for those that you do spot early, clear them out if you can.
It's one less worry, some easy era score, and possibly a gold and tech boost opportunity as well.
I often play together with a mate of mine who is on the emperor/immortal level as well, and he struggles just as bad with barbs because he doesn't take them serious enough until it's getting too late.
As soon as a camp is spotted near you, you keep a warrior or slinger between it and your city to scare off any scouts.
The first warrior is not for scouting, unless you want to take a rather big risk.