If you built a USS LWB recumbent with 26"/27"/700c wheels, then you can get away with standard gearing. I have an older USS LWB Linear with a 27" rear wheel and standard road bike drive. It has a gear range that is more than adequate. Later on Linear went to the 26" rear wheel and equipped the bikes with a 3 speed rear hub (8 speed rear derailleur system) and a single chainwheel in the front. No mid-drive needed. I would think that perfecting the USS steering mechanism would be far more complex. It has to be fairly tight with no play but not binding either. If the tie-rod system on my Linear was not properly adjusted, the bike would be very hard to control. When properly tightened with no slop, it required just a feather touch.

BTW, I don't think you can build a bike like the older Linear for what you can buy them used. I bought one of the older Kann Body Shop models made sometime in the 1980s for $400 from PoweronCycling in Florida a few years ago. It was perfectly rideable out of the box.

A fellow recumbent rider I know built his own recumbent a decade or so ago and only got a few rides on it before one of the brazed points failed. He never trusted it enough again to ride it after he strengthened the braze. It was built from parts he salvaged and a kit he obtained from someone in Washington state. It's beautiful but he now only rides the used recumbents he bought after that experience.