I agree with your assessment. LBJ washed away genuine accomplishments with Vietnam.
Nixon did the same with Watergate. No, his accomplishments weren't quite as large, but he didn't come into office with a 68-32 Senate and 295-140 House, either, did he?
(This is why I roll my eyes at the claims FDR and Lincoln were the greatest Presidents, the simplistic thinking involved. Give ANY President the control Lincoln had when the South seceded, and you can do all kinds of things with no Congressional opposition. Same with FDR, who had a 69-25 Senate after the 1934 midterms).
As far as the supposed claim of LBJ about "we've lost the South for 200 years" or the countermyth of Republicans saying he said, "I'll have those N-words voting Democratic for the next 200 years," we really need to get away from anecdotes as evidence of anything, EVEN IF THEY DID SAY IT. Humphrey won Texas in 1968, and Carter won it in 1976. Plus, Nixon PROBABLY would have won it in 1960 without LBJ, and nobody outside of AOC can try to seriously argue Nixon would have won in 1960 because of backlash in 1968 (which didn't help him win it that year, either). Eisenhower won Texas twice and Hoover won it back when it was solid Democratic territory in 1928.
I'd say Johnson was accepting reality that, yes, included the CRA, but there was a LOT more to it than two civil rights laws, too. John Tower won LBJ's seat in a 1961 special election and became the first Republican elected by the people to a Senate seat in Texas. Nobody with a brain can say, "Tower won in 1961 because of laws signed in 1964 and 1965." Plus, everyone had their eyes on this new Congresscritter from Houston, a man who worked well with both parties, so much so that the Democrats did not even put up a candidate against him in his 1968 House race.
Sadly, we never got to learn how far George H.W. Bush could go in Texas politics......