One thing I have seen done is to force a 1/4"-20 stainless steel stud into the stripped hole. I/4" is slightly larger than the M6 and will cut threads as you screw it in. I have not tried this myself but Andy from the Off-Grid-Garage channel has shown it several times.
He explains it starting at 7:35 in this video.
I have drilled and tapped 1/4" threads but if I need to do this again, I will try just forcing it in.
Others have used spiral inserts, I have tried that but never seemed to have as good of a result as others.
BTW: A few things about this.
* I much prefer the cells that have studs welded on.
* Every time the stud is threaded in and out of the terminal, the stainless steel stud will damage the soft aluminum of the terminal. When you get the cells, screw in studs with red locktite and never take them out again. (Make sure you use studs that are long enough to hold the busbars, lock washers lugs and nuts.)
EDIT: Corrected the statement to say tighten the nuts (not the studs)
* A torque wrench is a good (must have?) tool. Tighten the
nuts to 4.5 N-m. This is the one I picked up:
EDIT: Corrected the link