Hebrews 10:21 (CSB) and since we have a great high priest over the house of God,

Jesus is that high priest! Hebrews tells us that He is a priest in the same manner as Melchizedek. Why is Jesus not a priest in the order of Aaron? After all Aaron and all his children was instituted by God to be priest. First Jesus was from Judah (David) and not Levite (Aaron). Second Levities were just men who did not worship the Golden calf but latter turned to other gods. Melchizedek was the king of Salem (peace). He was a priest of God. He was not instituted by anyone to be priest but only grew into that position due to his Love of God. Aaron and his sons were priests by their heritage. Melchizedek was priest by his devotion. I don't think it is by accident that Melchizedek was the king of peace and Jesus is the prince of peace (Isaiah 9:6).

John 14:27 (CSB)
"Peace I leave with you. My peace I give to you. I do not give to you as the world gives. Don't let your heart be troubled or fearful.

Isaiah 26:3 (CSB) You will keep the mind that is dependent on you in perfect peace, for it is trusting in you.