What you’ve described is almost exactly what I did in my bus with wood glue and pocket screws. I used dimensional 2x2 as it’s stronger than the framing stuff. It has held up over more than 6,000 miles now. The 3/4 ply on top is overkill in my opinion. Use a good quality ACX and you’ll be fine with .5”. I used .5” on top and bottom of the 2x2 frame. There’s been no sag and I’d guess it’s holding 60-70 lbs plus the forces of driving.