So having a “will and ‘good pleasure’ that has not changed since ‘before the foundations of the world’” would be the same as having a fixed set of rules to make judgments by?
All whim and arbitrary come down to are not having a fixed criteria. Does God have a fixed criteria for deciding who is Elect and who is not?
I am sorry that you have not found my answer satisfactory. All I can do (and am willing to do) is go as far as the Scriptures go. God has not chosen to reveal the precise criteria He uses for "His pleasure." From what He has revealed of His nature, I accept that His motives are in character with His nature - as a just and holy God. I would point out that a God of whom a finite human could have exhaustive knowledge would no longer be infinite. I submit that God has been explicit about His character and motives to give us sufficient justification to conclude that He is neither capricious or whimsical. Note that I said sufficient not exhaustive. I would humbly ask anyone not satisfied by the answer of the Scriptures to examine their motives and ask if they were trying to judge God. That is the core of Paul's answer to your objection in Rom 9:19-22. I do so as one forgiven for previously holding such an attitude. I am thankful that He taught me from the Scriptures not to do so. I am thankful that He has been patient and longsuffering as He works on my attitude.