I've been wanting to do something about the dinette too but all the crap under there is a real pain.
I've been considering a RecPro or Thomas Payne jackknife sofa to make a U-shape seating area like you've done. They sell them without legs so it could just span the gap over the benches and I wouldn't need to build anything. It's 74" wall to wall and RecPro has a 72" sofa
that looks like it would fit really well. The Thomas Payne one comes in 68"
which is pretty close and I like the fold-down console in the middle.
The remaining ends of the benches would just need smaller cushions to make a U-shape seating area. As a bonus the sofa flattens out into a bed so you still have a sleeping position there if needed.