BTW-Isaiah 6:8 is frequently quoted in missions to save the lost. However, this was not Isaiah’s mission according to Isa 6:9-11. It was exactly opposite.
But you can’t deny that God’s purpose through Isaiah was Israel’s salvation. The whole book is God begging Israel to get right. But God, being who He is, knew they would reject Him. It’s really that simple. God knows who will reject Him and who will accept Him, but His desire is that all are saved.
Isaiah 55:6&7
Seek ye the LORD while he may be found, call ye upon him while he is near: Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the LORD, and he will have mercy upon him; and to our God, for he will abundantly pardon.
Isa 49:15
Can a woman forget her sucking child, that she should not have compassion on the son of her womb? yea, they may forget, yet will I not forget thee.