Just started Chapter 5, which opens up the city of Baldur's Gate. Have TotSC installed, so when should I go to Ulgoth's?

I chose to go at the last possible second, but you can go just about anytime. Now's a good time to go if you don't mind a challenge, but if you wait for a few more chapters, things might get a bit easier.

(PS: I'm hope I'm not too cryptic with "last possible second", but right when you think the game's wrapping up, there's a point where you could go run off to Ulgoth's, and come back and pick the game back up like you never left. ;) )

I normally go to Ulgoths Beard when I come back from my second 'visit' to Candlekeep.

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Insane

you can go to the town whenever you like - its a town like any other and towns have nice things like shops (to take an example at random ;) )

the quests are pretty hard so as late as reasonably possible is a good plan

well you can easily do Ice Island any time you want, it's pretty easy