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.