How do non-Catholics understand Catholics' fulfillment of the Great Commission?

Most translations of Matthew 28:19 begin with

“Go therefore and make disciples of all the nations, baptizing them …

What "of" means here is ambiguous:

• all people from all nations should be baptized. (Proselytize)
• a few people from each nation should be baptized. (Evangelize)

It is often assumed to have the first meaning, a mission to convert the entire world.

But if that is the case, why has the commission failed so badly? After 2000 years, most of the world still rejects Jesus's message.

Instead, Matthew 28:19 could mean to spread the word as far as possible, accepting those few converts that God calls to serve as examples throughout the world, even though the vast majority of the world will remain unconverted.

Consider the parallel passages:

And He said to them, “Go into all the world and preach the gospel to every creature.
— Mark 16:15

… repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.
— Luke 24:47

“But you shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth.”
— Acts 1:8

They say "preach the gospel to every creature", "preached in His name to all nations", "be witnesses to Me … to the end of the earth".

These all confirm the idea of bearing witness and preaching the gospel throughout the world. But they do not state, or even imply, that the whole world will believe and become converted Christians.

The idea of "the Great Commission" to convert the world to Christianity is an extra-biblical concept.

The Bible instead teaches that until Jesus returns, the number of true Christian converts will be very small; it is Satan's false version of Christianity that will dominate.

Jesus predicted that at the time of the end, just before his return, Christians would be a persecuted minority:

… they will deliver you up to tribulation and kill you, and you will be hated by all nations for My name’s sake. — Matthew 24:9

But before it is silenced, the true Church will spread the message to the world:

… this gospel of the kingdom will be preached in all the world as a witness to all the nations, and then the end will come.

Consider the "two witnesses":

Then those from the peoples, tribes, tongues, and nations will see their dead bodies three-and-a-half days, and not allow their dead bodies to be put into graves. And those who dwell on the earth will rejoice over them, make merry, and send gifts to one another, because these two prophets tormented those who dwell on the earth.
— Revelation 11:9–10

It is only with the advent of radio that a massive blanketing of the world with the Gospel has become possible. And only with the advent of the internet that the whole world can observe the events happening in the streets of Jerusalem.

If there is a "Great Commission", it is to make the world aware of the gospel, so that when the end arrives people will recognize that what they had heard wasn't simply nonsense, and they will turn to God.

OP: "Since Catholic's have done the best job fulfilling Christ's command, doesn't this mean they are closest to Christ's teaching and thusly the true Church?"

The problem to be solved is proving that today's Catholic Church is identical, in faith and practice, with the church in Rome, or Ephesus, or Acts. It is very simple to say its doctrines have changed over the centuries. For example, the Marian dogmas are defined very late, yet are considered de fide, which is something the church at Rome that tried to "teach the same" per Jude wouldn't recognize.

In other words, the question or assertion is a non-starter.

Besides, if majority means right, why did Elijah solely come against the 450 prophets of Baal?

So, "the Catholic's" whoever that might be over the last 2,000 years have no claim per se in fulfilling the great commission or are closest to Christ's teaching or are the true Church. Prove that for 2,000 years nothing has been added to or subtracted from that faith once delivered.

Having said that, who has contended for said faith over 2,000 years?

Since Catholic's have done the best job fulfilling Christ's command, doesn't this mean they are closest to Christ's teaching and thusly the true Church?

More formally, the argument in the OP can be reworded as follows:

• P1: Catholics have done the best job fulfilling Christ's command
• C1: Therefore, Catholics are closest to Christ's teaching (from P1)
• C2: Therefore, Catholics are the true Church (from P1 and C1)

I see at least two ways to defeat this argument.

Option 1: defeat P1

You could argue that there is no compelling reason to accept premise 1. For example, high quantity of conversions does not guarantee high quality of conversions. Even demons believe and tremble. Getting lots of people to accept religious beliefs does not logically entail getting lots of people to become true disciples of Christ. In order to accept premise 1, you would need to show that Catholicism produces more true disciples of Christ (not just mere believers) than other denominations. Since the OP didn't offer compelling arguments to make this case, I see no reason to accept P1.

Option 2: show that C1 and C2 do not follow from P1

If we concede premise 1, it doesn't follow that true disciples of Christ are not being produced by other denominations as well. Perhaps other denominations are producing true disciples of Christ too, albeit in smaller numbers. Those true disciples of Christ would be part of the true Church, and therefore, this would contradict the conclusion that Catholics are the true Church (if we interpret are as denoting identity (A = B)).

A more defensible conclusion that could be drawn from P1 would be that Catholics are statistically more likely to be part of the true Church, leaving room for the possibility that other true disciples of Christ can be found outside of Catholicism.

Now, in order to show that Catholics are the true Church (in the sense of identity), you need to show that a person is part of the true Church if and only if they are a Catholic, but then I fail to see how this conclusion could possibly follow from P1. You would need a stronger set of premises (and stronger arguments for said premises) to derive this conclusion.