Is there a dimmer light switch with secondary constant 120V output?
Check out lutron SFSQ-LF-GR 120v 360w 1.5 amp 3 speed fan control light switch on 1 yoke this will fit in your existing space since that was your concern. I may have mis read your question, so here is a light dimmer with a separate on off switch for the fan, Lutein MA-L3S25-WH. Hope one of those works. Or a pir sensor and switch so the fan turns on when you enter and a light switch combo but those are really expensive.