You are right about the part time jobs suiting employers, they would rather pay two pt workers than one ft so they can avoid tax payments.

The other thing I find (in UK in any case) is they may offer pt contract but then they expect you do more hours when it suits them then they cut back to contract when they want to. Not helpful to you but that's the reality. I find it helpful in my min wage job to keep good boundaries - this stuff used to stress me out but a lot less recently.

Enjoy your volunteering, it's much nicer than paid work in my experience.