My new 6040 improvement plan.

[m_c]

The biggest problem is that you'd be bolting the rails to aluminium, not the bolts themselves.

There are various calculators online for bolt strength, and the first one I've just found puts an 8.8 grade M3 bolt at proof load of 2920N, and minimum ultimate load at 4020N. To put that in context, that means you could hang over 400Kg onto the bolt vertically, and it should not snap.
However I very much doubt an aluminium thread would support that 400kg.

I'd be looking at either helicoils, or bigger threaded inserts which I've currently forgotten the name off, to help spread the load on the aluminium.

[PaisleyPCdoctor]

The thought did occur to me about load on threads, albeit not as precisely as you.

The way it's looking the rails will probably be mounted with trucks facing up and down (above and below the existing rods, so load will be perpendicular to the bolts, rather than trying to pull them out, if that makes sense.

If I end up leaving the 20mm rods in, it shouldnt be an issue. The load will be shared in some fashion between 4 bearings (or six if there's space to double up on the square rail trucks).

I've ordered a single 15mm hiwin clone from uk with 2 carriages. It'll be 100x easier to visualise a working plan with them in front of me.

Also- might have mentioned- to avoid the definite coking up of manually drilling all those mounting holes- I'm gonna use a bit of 2020 extrusion between rail and aluminium backplate. That seems less rigid but at least I can do it.