I would like to slow the timescale. Are there any quests broken by this?

I don′t think so. FWE does that and I have not hear about problems with any quests using this mod.

I don't use the default setting (mine is set at 3) & I haven't noticed or heard of issues about it

I tried estimating the 'correct' timescale and worked it out as roughly 15, and have had the game set to that for certainly more than 100 hours of play without any obvious problems.