Isaiah 9:6 is clearly about Hezekiah. Mighty god "el gibbor" is a title given to Jewish heroes and great men. It says- a child has been born to us, a son has been given. There's nothing future tense about it.
As God declared the end from the beginning, I don't see that the tense means what you believe it does.
Could you explain how of the increase of Hezekiah's government there will be no end?
That's an awful lot of titles and importance spoken of -why is he the focus of all of that and not others?
I agree that the subject matter preceding the part about the child being born seems to indicate the subject matter is things of those days -but abrupt changes in subject matter (and also duality of subject matter) are not uncommon in prophecy.
Prophecies also often cover a great amount of time.
For example... In the next chapter of Isaiah is a prophecy (a continuation of the same prophecy) of the Assyrian lifting up his staff and smiting those who dwell in Zion in times yet future -but that the end of the matter will be the destruction of the Assyrian....
Further proof that it is yet future is that the Assyrian has not yet been broken in God's land.......
God's Purpose against Assyria
Isaiah 14:
24The LORD of hosts has sworn saying, "Surely, just as I have intended so it has happened, and just as I have planned so it will stand,
25to break Assyria in My land, and I will trample him on My mountains. Then his yoke will be removed from them and his burden removed from their shoulder.
26"This is the plan devised against the whole earth; and this is the hand that is stretched out against all the nations.…
(This makes more sense when one realizes that the "Assyrian" left Nineveh for Haran -then continued NW to what is now Germany then known as the Chatti [described in Ezekiel 17] and are the same that troubled Jews in WWII -then referring to themselves as Aryan. It should also be noted that the prophecies are about "the Assyrian" -not Assyria -not a specific place, but a people.)
(It is said "Never again!" -but there will be one more attempt at a final solution. However, Judah will fare better than the house of Israel -who will experience similar to that of [primarily] Judah in WWII -unless they repent)
....this is the same event described in Daniel 8 and 11
And through his policie also hee shall cause craft to prosper in his hand, and hee shall magnifie himselfe in his heart, and by peace shal destroy many: he shall also stand vp against the prince of princes, but he shalbe broken without hand.
King James Version
And he shall plant the tabernacles of his palace between the seas in the glorious holy mountain; yet he shall come to his end, and none shall help him.
...and much of Daniel is definitely about the end times
…
8As for me, I heard but could not understand; so I said, "My lord, what will be the outcome of these events?"
9He said, "Go your way, Daniel, for these words are concealed and sealed up until the end time.
10"Many will be purged, purified and refined, but the wicked will act wickedly; and none of the wicked will understand, but those who have insight will understand.…