Been there. Where it helps is in having all this extra time for understanding what you actually want from life, and crucially also job search and interviewing (whilst the employer is under false impression you're now more loyal). Side projects - not so much. The thing is, those fundamentals that are causing the bad mental state stay the same no matter how many hours you work, shorter work week allows to disengage a bit more but that's just postponing the only working solution which is quitting.