I did it post final boss, the last defense part is in the final area too. But do whatever you want.

i would say do the rest of the base game and try to get to lvl 90+, every possible quartz/amuletts and defence parts else you will have a bad time in the dlc

Started it at about level 70, as soon as I was able to, and have had a good time clearing it.

Mind you, I've completed the base game multiple times and know the mechanics.

Personally, the best you can do, is get to the dlc at chapter 9 in NG. But before entering, go down the elevator to the Trismegistus Cave, kill the first "mini boss" to get the increased physical defense amulet. And then go back to Hotel and get to dlc.

Don't even activate the stargazer so you don't advance to chapter 10 (I don't think it matters though).

Or that's what I did, at least.

Cheers!

I'd say the perfect place is stopping somwhere between the first and third stargazer on the final island. You'll get some relevant story simply by going there. Plus you'll cement your build and level up plenty by then.