US/UM didn't chart till day 56 so you can't compare them yet.
3ds had over 5 times the install base of Switch in Japan when these two titles released, as well as the fact US/UM is apart of the main core series formula while Let's Go isn't. There is no big reason to believe Let's Go will have a bigger opening than US/UM had due to that. Lifetime possibly, but even if this a proper Gen 8 game with sheer install base difference would make it a reasonable feat.
I believe Let's Go will open with 800k-900k which will be fine. People have to remember this is a remake of an enhancement (Yellow) and is heavily influenced by Go. In fact, Yellow was released during the peak of Pokemania yet "only" opened with 850k.