You have referenced Hebrews 5:8-9 several times, and certainly they are intriguing and challenging verses. The question is, as I have written before, how does the Christ, who 'knows all things' (John 21:17) need to learn anything? And, specifically, why does the One who was obedient in His childhood (Luke 2:51) and all through His ministry (John 5:30; Philippians 2:8), need to learn obedience?
Here's my take on it.
I have a friend I made on an internet forum who is Pastor of a small church in Texas. He married a lady in his youth and at that time, if you had asked him if he understood the bit in the marriage vows about loving, 'in sickness and in health' and 'forsaking all others,' I expect that he would have angrily insisted that he did.
However, in the very first year of their marriage, his wife picked up a viral brain infection and became totally unable to move or respond to him in any way. She continued in this state for over 20 years until she died a few years ago. During this time he continued to be entirely devoted to her and never went with another woman.
Now, he knew theoretically what the marriage vows meant, and when it came to the crunch he kept them, but now he knows practically what they mean, and no one can ever tell him that he doesn't because he has lived them out.
So in the same way, when we pray to God and say, God, "I'm having so much trouble living out this obedience to Your Lordship, and resisting sin," He is not going to say, "Well, I don't understand any of that. I've never obeyed anyone ever and I cannot be tempted to sin." No! our Lord Jesus Christ knows from experience exactly what obedience means because He lived it out, even unto death, during His time on earth, and He was tempted or tested in every way, just as we are (yet without sin). So 'We do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted just as we are, yet without sin [so] let us therefore come boldly to the throne of grace, that we may obtain mercy and find grace in time of need' (Hebrews 4:15-16, just a few verses before 5:8-9).
But this does not impact upon His cross work which is described in Hebrews 9 & 10. It is as well as, not instead of, His work of propitiation. There are two parts to the work of a high priest: to offer sacrifices, and to intercede for the people. Christ, by His perfect substitutionary sacrifice of Himself on the cross has taken away the sins of those for whom He died, and, just as the High Priest carried the names of the 12 tribes of Israel over his heart (Exodus 28:29), so Christ ever lives to intercede for us, knowing from personal experience our trials, temptations and sufferings.
Alleluia! What a perfect and wonderful Saviour we have!