My Opinion: You should ditch the ErgoRest idea.
I feel like I started at a similar place to where you’re at now:
Arm RESTS are called arm “rests” for a reason. They are meant for when you are passive. You should be switching from hovering over keys typing to thinking with arms rested and back again as a optimal workflow.
While there are studies on the benefits to arm-rest-mounted keyboard halves, there is not study on the long-term effects of resting all your weight on one part of your arm for hours at a time days at a time.
I would recommend getting to “natural” as a position as possible unless you have special needs due to some kind of injury (consult a doctor or professional).
For me I keep hearing the words repeated at an national Ergonmics conference I crashed recently:
“Your next position is your best position”.
I feel articulated arms is the near-term solution to this problem, and can imagine a future where these arms are automated just like sit/stand desks. In fact, I wish my sit/stand had a scheduler so I could schedule different times of the day to sit/stand.
At any rate, I would shy away from locking your arms and torso in any one position unless you have a physical restraint to do so. Aim for something that will maximize your movement throughout the day (I saw a YouTube review on ErgoRest that the user felt it was hard to move and restricting his movement).