So tell me, in the Greek NT, why is there no word for “only” and why is the word for “also” bracketed with notes that it’s omitted in a bunch of witnesses?

Matthew, Did you happen to see the comments of the NET Bible translators regarding the presence of “also” (kai):

The word “only” is not in the Greek text, but is implied by the ajllaV kai (alla kai) in the second clause (“but…as well”). The bulk of the Western text dropped the kaiv, motivated most likely by ascetic concerns.

I think whether or not the “also” was dropped or added, the text is still a great statement of what Kingdom life is, and your interpretation is dead on. And considering the very next verse is “Have this attitude in yourselves which was also in Christ Jesus,” (that is one of selflessness and humility), verse 4 is definitely no less of a challenge even if Paul did write “but also..”