Please, never that left hand thing
I've been trying for years to convince my wife that the only drivers who remember what that raised left hand thing means are so old they can't see you anyway. After a while of refusing to admit I might have learned something in 30 years of cycling in traffic, she changed. She started putting her left hand up, pointing her index finger to the right. I finally laid into her about it, she got PO'd at me, but I'm trying to keep her alive.
Simple, large gestures for simple drivers of any size. Point left, point right, anyone not staring at their phone can figure that out. They won't know stop or slow, so I don't confuse them with it. At an intersection, I'll sometimes point straight ahead if I see they are looking for some indication from me.
There are other signals for bikes following me, but I assume they should know more than drivers about what I'm saying.