Thread repair and minimum thread insertion depth for steel bolts in aluminium frame?

Hi all,

I am having issues with my Canyon Commuter bicycle. The seat stay rack mounts threads have stripped on the non drive side. The way the rack was secured is an M5 steel bolt into the aluminium frame. However the way it was installed from the factory means that only 3mm of threads from the bolt were secured in the frame. This just seems to me like it is destined for failure. Does anyone know what the minimum insertion depth for a strong connection is? 3mm on an M5 steel bolt in aluminium just doesn't seem enough.

Is this repairable? I was thinking of order a thread repair kit and see if I can retap the threads. Would you try with an M5 thread first or go bigger straight away? or would a helical be better? Theres just not much meat in the seat stays and I do not want to screw it up so it's unusable.

cheers

Stu