Will be interested to hear more experienced feedback...but...

My old mdf machine used linear bearings made from 1/8" (3.2mm) aluminium angle with skate bearings - I was using M8 bolts & tapping the aluminium...that held very well.
Based on that experience alone I was thinking of using M8 for my new build on 3mm steel & M10 where I have 4mm steel to play with.

Lets see what the elders say...