Why did Paul Thank God he didn't baptize some individuals?

17: For Christ did not send me to baptize, but to preach the gospel--not with wisdom and eloquence, lest the cross of Christ be emptied of its power. - 1 Corinthians 1:14-17

I think the question that must be asked is, why does Paul consider the gospel more important than baptism? And why is baptism missing from the verse below as well every other reference in scripture referring to eternal life?

Eph 1:13 In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise,

15 Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, 2 by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain. 3 For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He was raised on the third day according to the Scriptures,

You brought up 2 references: Mark 16:16 and Acts 2:38. There are a few issues here if we take this to mean baptism is required for salvation.

1. This view is inconsistent with the rest of scripture. (Read the entire gospel of John and see how many times baptism is not mentioned.)
2. Mark 16 most likely ends after verse 8. Everything after verse 8 was not found in the early manuscripts and is in a different writing style than the rest of the book.
3. If Act 2:38 is saying baptism saves you then it's also saying you receive the Holy Spirit at this point. John 7:38, Acts 19:2, Eph. 1:13, all claim that the Holy Spirit is received at the moment of belief. Also, in Acts 10 the believers receive the Holy Spirit before baptism. Therefore there must be a better interpretive option to understand Acts 2:38

To answer your bullet point questions:

Does this mean

1. Paul didn't want to be glorified (v.15) - I believe it means Paul understood that people are saved when they believe in Jesus through the gospel. He would have understood that baptism in water is like a wedding ring. It's a public sign of something that already happened. Just like circumcision. Baptism in water doesn't wash away sins or save a person. Jesus washes away sins through the baptism of the Holy Spirit at the moment of belief.

2. Their faith was so strong that Paul did not need to baptize them? (i.e. is baptism considered a work in this context? ('lest the cross of Christ be emptied of its power.' or Ephesians 2:18)) - I wouldn't say so. If you read 1 Corinthians from cover to cover you will realize they are believers and that they struggled with sin, faith, and faithfulness.

3. They would get baptized later because of their strong faith & commitment to Christ - I don't think so. The pattern we see in Acts is that people would believe and then get baptized if able. It has nothing to do with the strength of their faithfulness.

4. Something else? How should this verse be interpreted? - refer to point 1.

In-other words, Is Paul fulfilling his duty as a disciple?

If he was sent to preach the gospel then, Yes. He absolutely did that. He also pushed people to make sure the object of their faith and salvation was in Jesus alone.

In this passage, St. Paul is cautioning against schisms among the faithful:

1 Corinthians 1:1-17 (DRB)

Now I beseech you, brethren, by the name of our Lord Jesus Christ, that you all speak the same thing, and that there be no schisms among you; but that you be perfect in the same mind, and in the same judgment. 11 For it hath been signified unto me, my brethren, of you, by them that are of the house of Chloe, that there are contentions among you. 12 Now this I say, that every one of you saith: I indeed am of Paul; and I am of Apollo; and I am of Cephas; and I of Christ. 13 Is Christ divided? Was Paul then crucified for you? or were you baptized in the name of Paul? 14 I give God thanks, that I baptized none of you but Crispus and Caius; 15 Lest any should say that you were baptized in my name. 16 And I baptized also the household of Stephanus; besides, I know not whether I baptized any other. 17 For Christ sent me not to baptize, but to preach the gospel: not in wisdom of speech, lest the cross of Christ should be made void.

He thanks God he baptized not a lot of them because it means they can't involve him in their strife and schism, attributing their faith and justification to the ones who preached to or baptized them—'fathered in the gospel' (1 Cor. 4:15): "Were you baptized in the name of Paul?" Christ is not divided, and so there shouldn't be schisms among the Body of Christ, but unity of mind and spirit.

He obviously didn't mean, 'what use is baptism? preach the gospel instead,' since the Great Comission is to "disciple all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost," (Mt. 28:19) and "baptism now saves you" (1 Pet. 3:21), being the "circumcised with a circumcision not made by hands ... having been buried with him in baptism, in which also you are risen again by the faith of the operation of God, who hath raised him up from the dead" (Col. 2:11-12), and is in fact one of "the very first rudiments of Christ" along with the resurrection of the dead, faith, repentance, laying on of hands, etc. (Heb. 6:2; Acts 8:35-36). He means rather, 'my apostolic mission is to be an evangelist, and not a baptizer of neophytes.' There are different roles for different people in the body, and Paul's primary one was to make manifest the gospel in its fulness, and not to spend his time instead baptizing (when there are others who can do this), and leaving the knowledge of the gospel which makes the baptism performed of any significance or meaning. The gospel is the basis of baptism. "Without faith it is impossible to please God" (Heb. 11:6).

The church of Corinth had a problem: it was split up into quarrelling factions. The people were more focused on the differences between them, such as which apostles they identified with, than the unity they should have had as fellow members of the body of Christ. Many people "followed Paul", but evidently few had been personally baptised by him.

Paul is thankful that he didn't baptise many of them not because he thought they shouldn't have been baptised, but because it might have made this factionalism even worse. If a large number had been personally baptised by Paul then they surely would've been lording it over the others.