Most likely you will find a big improvement (less wobble) with a little more distance... I believe that when you begin riding bents you tend to tense up a little bit and this exagerates steering movement.
With some more distance you will relax and things should smooth out, this does assume that the wobble isn't caused by a mechanical problem or a poorly designed bike... since you said you tried several Rans recumbents, I doubt it's a problem with the bike.
I find looking well ahead seems to help with keeping things straight.
Get some more miles in... relax and enjoy.