Need ideas (or someone to make) a widget to couple motor shaft to M8/M10 threaded rod

[Ross77]

The problem with a bearing on both ends of the threaded rod is one of access...

The bearings don't have to be at the very ends. You just need the shaft constrained by two bearings to keep it concentric, then the flexible coupling can do it's job and take out any minor alignment problems. keep the arrangement you have but add another support plate and bearing after the coupling, as you have it at the moment the threaded bar will pivot about the middle bearing and exaggerate any alignment issues.

Have you thought about using a hex bolt and cutting the head off? that way the shank could act as a shaft for the bearings and having a solid round end would be better for forcing on a under sized flex mount.