I find it is all but impossible to exhaust the Lord’s parables. There are always more principles to learn from them and new ways they might legitimately be applied. So don’t mistake the following for an attempt to fully exposit Matthew 18:21-35. I am just nibbling around the edges.
What I do find useful is to work my way through the parable eliminating the obvious. Once that is done, I can give slightly-less-confused consideration to the possibilities that remain.