My feeling's that at 120gsm the finished book won't have much 'presence' in the hand, & will seem too slim & floppy, even at 200pp.
But I thought your aim now was to reduce the page count, which tips the balance even more.
I'd be looking at 150 - 180gsm. Feel's important - both for ongoing personal satisfaction but also for first impressions as people pick one up somewhere with a view to buying one.
Yes a heavier paper will be stiffer, which impinges towards the spine, but would make the book more of a 'thing' in its own right, & something worth having.
Just my ten bob's worth. Do you want my sort code?