I watched Miami since the 90's.
Yes, Jimmy failed to bring them to the superbowl.
Yes, he couldn't replicate finding another Emmitt Smith in the draft.
When Shula was there, they let Marino run the offense. Johnson wanted a stronger run game and he clashed with Marino often. Yatil Green and John Avery as 1-2 misses at RB and WR hurt.
However, he did a great job building up the defense. Like you said, Dave had a great cast to work with on that side, but a mix of QB purgatory and still bad drafts doomed the Dolphins.
IIRC Johnson wanted to trade Marino, but he was a big name down here. And Marino was still a good QB but injuries were piling up and while he still had an arm he was a statue. And his last year coaching in Miami he was checked out after his mom died in December of 1998. He resigned in Spring of 1999, but was talked back into the 1999-2000 season.