You can replace 8mm bolts with AN5 aircraft bolts. You can get them at aircraftspruce.com or many other sources. Stronger than Grade 8 and not as brittle. Their threaded portions are short, so no need to trim excess.
If you stay metric, make sure you are using grade 12.9 (the strong ones) and get them from a reputable source like mcmaster.com
That said, if you are breaking bolts, there is probably something wrong with the mount or the cases. Racing days are rare and it sucks when your stuff breaks. I’d probably replace the mount first as long as the cases look okay. If not, replace both.