I don't use r/c motor sprays, I use shellite as well. Just remember that it dries out the lubricant in brushes so if you run the motor after you've just cleaned it then the brushes will overheat.
But you're 30 min gap should be fine.
From what you've said about the hardness of the brushes it would seem this is the problem.
Have suffered virtually no wear from a good number of runs
Considering you are running 8 turn motors I would say that such long life shouldn't be the case.
I know these new brushes are designed for longer life, but if you dramatically increase the life of the brush, it is usually at the expense of comm life. If it was me I'd be trying every brush I could lay my hands on until I found one that is more "comm friendly".