Your use of fullness is very good:The number 144,000 is intentional and signifies "fullness that is immeasurable".
Rom 11:25
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
Rom 11:26
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: For this is my covenant unto them, when I shall take away their sins.
However, the use of immeasurable isn't necessarily true. At least not the same as the innumerable souls later in Rev 7, that no man can number.
Rather instead, 144,000 is exactly as you say, the fulness of the first resurrected saints of God, which will be a great yet finite number.
And at this point, since that number is not yet finished until the Lord returns, then 144,000 is perfect use for prophecy. I.e. the prophecy will be fulfilled, that the fulness of the Gentiles and Jews engrafted into the Israel of God, shall all be saved, resurrected, and stand with the Lamb on His Mt Sion.
We have
12 (the number of government)
x
12 (this may also signify perfect governmental rule)
x
1000 (perfection)
It's not a coincidence that is what John calls what He sees: "..a great multitude no one can count."
I'm still convinced by simple grammar alone, that the fulness of 144,000 is still a number, and so cannot be the later great multitude, that has no number at all...
Rev 7 begins with the sealing of the first resurrected saints on earth, and concludes with the innumerable souls dwelling with God and the Lamb in New Jerusalem on the new earth. Rev 14 is the 144,00 first resurrected saints raptured to meet the Lamb in the air on Mt. Sion.
Correct. Which in Scripture is Sion.
Rom 9:33
As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed.
Rom 11:26
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: For this is my covenant unto them, when I shall take away their sins.
Heb 12:22
But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels,
Rev 14:1
And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads.