Before this question can be answered, the identity of this group of 144,000 people of all the tribes of Israel, said to be sealed unto God with his name in their foreheads (chapter 7) must be made clear. It is not until chapter 14 that we get more details, now they are in heaven. Their symbolic sealing is repeated, they are singing a new song, unique to them, virgins etc, then comes the bit you ask about - What does their being 'firstfruits' to God and to the Lamb mean?
Given that the whole book of Revelation is symbolic, and that consistency in interpretation of the visions is required, we can assume that the number is not literal, the 'sealing in their foreheads' is not literal, their being 'virgins' is not literal, and that they are not literally picked up and 'waved' in heaven, as how a firstfruit sheaf of wheat was, in old testament times.
You have provided your own interpretation, which means I'm wasting my time providing this one, but I shall simply quote from a book that gives a different one. It is established that Christ is the one who does this reaping, because they are his own - they are firstfruits unto him, the Lamb, and to God. He precedes the angel sent to reap the wicked vine of the earth, for trampling. Just before that happens, Christ's firstfruits are safely gathered in (Rev.14:14-20).
"As to the one hundred and forty-four thousand, this figure evidently
answers to twelve multiplied by twelve. Twelve being the number of the
patriarchs of the children of Israel, as well as of the apostles of
our Lord Jesus Christ, evidently the number points to the multiplied
elect from both old and new testaments.
Twelve is the number of the covenant. There are twelve gates in the
wall of the holy city, new Jerusalem, having the names of the twelve
tribes of the children of Israel. This indicates the covenant people
of God's choice out of every tribe obtaining entrance by grace into
the heavenly city, that is, through faith in the gospel preached
before unto Abraham their father.
Likewise the wall of the city had twelve foundations, and in them the
names of the twelve apostles of the Lamb. This is indicative of the
new testament, or new covenant, for by the twelve apostles the
doctrine of Christ was both preached and taught. Thus all the chosen
people of God under the new covenant continue steadfast in the
apostles' doctrine and fellowship, being built upon one foundation,
which is Jesus Christ. Such a people are multiplied under the
preaching of the word. Thus, as one together with their old testament
brethren, the complete number is signified by the allegorical figure
of one hundred and forty-four thousand." (The Revelation of Jesus
Christ, pp.343-344, John Metcalfe)
Of course, anyone taking that number as literal will dismiss all the foregoing. But before understanding what 'firstfruits' means, as applying to that group, their identity must be correctly established.
"The harvest is the end of the world. This concept cannot change. But
there are two distinct applications of that harvest... the gathering
of the redeemed is put first... Hence it is the firstfruits unto God
and to the Lamb, reaped in person by one like unto the Son of Man. For
these are his own firstfrutis out of the one great harvest... These
are the redeemed from among men, rapturously taken up from the earth
and brought home to be for ever with the Lord'... The other -
virtually simultaneously - was the remainder of the harvest, being
reaped by the angel, and consigned to the winepress of the wrath of
God. One harvest, the harvest at the end of the world, but two
reapings." (Ibid. pp.360-361)
This is where clarity needs to come as to the distinction between the old testament first-ripe sheaf brought in - a harbinger of the whole harvest, seven weeks later. (See pp 403-406 of that book.) Application is made to Jesus being cut off from the earth on the cross, and the gap until that Day of Pentecost.
"Whatever was true of that firstripe sheaf, must be true of the whole
harvest. Hence, as the firstripe sheaf was carefully gathered by being
put to the sickle, the harvest of that from which the firstripe sheaf
was taken must likewise be reaped... Rev.14:16; Mat.3:12 & 13:30. 'The
harvest is the end of the world', Matthew 13:39...
That another - and distinct - angel of judgment came out of the temple
which is in heaven, after 'the earth was reaped' Rev. 14:16; and
reaped by one like unto the Son of man as opposed to angels; and that
this other angel had in his hand - yet to be employed - another sharp
sickle, of the same kind as that with which one like unto the Son of
Man had already reaped the firstfruits, argues beyond all reasonable
doubt another, a further reaping, even in that same day of the
Lord.... This implies that the wheat harvest, as opposed to the
gathering of the grapes of the vine of the earth, is an entirely separate event: the two representing as different a form in
agriculture, as do the sheep and the goats in nature." (Ibid.
pp.406-407)
Summary: Jesus' being lifted up from the earth on the cross, then going to heaven, represented the first sheaf cut as a guarantee of the whole harvest that would take place seven weeks later. Christ starts the one harvest at the end of the world by gathering to himself all who belong to him, immediately before an angel does the second, fatal reaping of the vine of the earth. One harvest, two reapings. The one hundred and forty-four thousand symbolise all the redeemed who are safely gathered in, to be presented before God and Christ in heaven, just before the wicked are gathered, trampled, and burned.
