Looks pretty good! I would add some support in the middle to stiffen the structure. Looks like it would be quite easy to bolt prototypes to the top of it for a test bed!
May not have to if they make plates that close the front and back up nicely (instead of the extrusion connecting the whole thing). It may be a little heavier but the flex would be minimal (if any).
What was the thought process behind the 8" wheels? Are those 8" wheels?
Yes, why? 8" wheels with maybe 0.5" ground clearance seems counter productive. You could have just as easily used 4" wheels with the amount of ground clearance you have.