I'm assuming you want this for range use only? It's unlikely you'll find parts online. At one time there were Performance Center versions of the 5906 intended for PPC competition that MAY have been single action like the 952 and 745. There are a couple of retired S&W armorers here who might be able to chip in.

A lighter trigger return spring plus some additional practice will help with that initial DA trigger press. FWIW, the minimum SA pull specification is 4.5 lbs.