Hey Dan,
Jacqui here from Sopo Bike Co-op. We have also been struggling with this issue in our shop, as Ryne mentioned. It is a great idea to connect with each other and figure it out!
I am also the only part-time employee of the shop, which is also all volunteer led.

I am working on an in-shop guide for volunteers that I will share once it is scanned in.
We are offering free volunteer trainings about once a month right now, but it would be ideal for us to have this bi-weekly or more.
I suggest providing free classes to volunteers and then offering advanced/specialized courses for a suggested donation, where perhaps volunteers can work on their own projects.

We have 2 professional mechanics who are volunteering their time to work with volunteers. We offer a basic "tool review" for newly orientated volunteers and then a rotating subject depending on volunteer requests.
We have offered: wheel basics, cranks and bottom brackets, and tool reviews so far.

Feel free to contact me by email or phone if you wanna talk about any specifics
p- 386 216 0346
e- jd@sopobikes.org