I believe they take place after the events of Sarek (which IIRC picks up immediately after STVI), where the crew get a reprieve from their decommissioning.
You're remembering wrong in a couple of ways. None of the first three TUC novels --
Best Destiny in 1992,
Shadows on the Sun in '93, and
Sarek in '94 -- were in continuity with each other. This was the height of the era when novels were required to have no continuity between them, except for the occasional direct sequel like
Best Destiny was to
Final Frontier. So each one is a totally independent and incompatible followup to TUC, and each handles the retirement of the
Enterprise differently.
Best Destiny specifically reverses the retirement at the end (a change I was always surprised Diane Carey was allowed to get away with).
Shadows on the Sun has the retirement take effect on schedule. And
Sarek ignores the issue altogether and just has the
Enterprise still in service post-TUC with zero explanation. And of course, later books like
The Ashes of Eden, The Fearful Summons, and
The Last Roundup are also out of continuity with the first three and each other.
For a while, I figured the first three books could be reconciled by assuming that the reprieve at the end of BD was temporary, that
Sarek took place while it was in effect, and that the reprieve was then rescinded between
Sarek and
Shadows -- so I had to mix up the publication order to make it work.