You'll hear folks say that the 12V conversion is hard on the Bendix because the starter spins faster. There might be something to this as I did in fact break a Bendix when I converted to 12V!
I wound up installing a different starter drive head as I read they didn't break:
https://cgfordparts.com/starter-driv...ml#description
They are a bit fiddly to install (you need to compress them to fit) and have a 'rattly' sound, but so far so good.