"frame connectors that are fully welded in place and are 1 5/8 inches x .118-inch MS or .083-inch CM round and/or 2 inch x 2 inch x .058 MS or CM rectangular"

I don't understand this. The connector provides vertical bending resistance.
The 2" rectangular is much stiffer than the 1-5/8" tube.

I fail to see the engineering that requires the rear tube to be welded to the hoop. A bolt-together design might be more expensive, complex, and heavy but easily made stronger than a weld. One method would be a flange-to-tube section welded to each of the main and rear, with a removable flanged tube between them. The attachment to both tubes passes already, and the shear strength at the flanges can be met by sizing the bolt area.
The welded area is the contact surface is (OD + ID ÷ 2) of the smaller tube × Pi × wall thickness. A 1-3/4" × .120" tube welded seam is .614 sq. in. 4 × 1/2" bolts provide .785".

Please: I'm OLD! Correct my errors, please.