And...my toothbrush is fixed thanks to a spare part from @rsim's stash of dead sonicares. Very easy to replace so I saved cash and kept it out of the landfill. Woot!
Time spent on repair - 5 minutes
Money saved (cost of new brush) - $80
What I learned:
- rechargeable battery inside should be replaceable (often it's glued it...but not heavily)
- the metal brush shaft is a very common break (this was my problem)
- board is well made and well-soldered by the looks of things
- no parts to buy online, you need another dead sonicare (people often toss them with the battery stops charging)
- don't drop the toothbrush!
Part that sheared off looks similar to this (photo from instrucatables)