Those front motor mounts look like a good place to attach a bracket to support the lower front of subframe.
I'd also consider moving the whole subframe closer to the center of bike, less flex on the mounting points.
I'm not a big fan of the U-bolts around the frame for the upper front mount, put a lot of stress in a narrow area. If nothing else could you find a piece of tubing, say 3mm or thicker, large enough on the inside to wrap half way around the down tube to spread the load out?
One more idea. Use a section of the blue tubing as a spacer between the upper end of subframe and the down tube. This would put the threaded rod under tension load and less bending loads.
Guess I should be working on my XS400 subframe instead of telling you how to build yours!