I too was fulltime during my premed path and can appreciate how challenging it is to break into the field without having an 'in'.
I asked my PCP and it was no big deal for me to follow him around for an afternoon. After that I applied to all the hospitals in town that advertised a formal volunteering program on their website. In the end, it was on me to physically call these places and get my voice in the ear of the volunteer coordinator. That eventually led to another ER shadowing experience but not volunteering because that required a university affiliation (at least in the ER). I really liked the sum total of these shadowing experiences, so at that point felt comfortable enough to begin the online prereq coursework. I then pretty quickly landed a volunteering opportunity after becoming affiliated with a university.
So to condense, my recommendation is to begin volunteering asap. Shadow if you can't land a volunteering gig, but only for as many hours as is needed for you to determine that medicine is something you need to pursue. And please, run if it isn't something you need to do! Then volunteer during your coursework. Sustained volunteering is really important both for you and your eventual application to medical schools. You'll inevitably circle back to do more shadowing if you end up applying to medical schools.