You are correct, you can only step back and then step forward with ACR. You cannot take two (or more) steps back. There is no ability to do continuous stepping back in ACR so there is no way to change key commands to do so.
You might consider using Lightroom Classic which is "ACR with a database." With LR-Classic, you can go all the way back to the first action you took on an image even if you last worked on that image a year or more ago.