I assume that "L20" is a trade name of some sort. Mild steel (assuming this something like 1020-1018, will work fine for single triggers case hardened or otherwise. Case hardened will be "slicker" and will resist corrosion better and cane be done in colors or cased then repolished & heat blued.
You mention using 1% carbon steel, this would be very high carbon steel like 1095.
I tend to make triggers and other parts from 1018 I get from MSC. No scale, has a ground surface, works easily cases well and I get the exact size I want.
Dan
OOPs didn't read everything. Yeah 1020.