Perhaps after the Dominion War? Would Seven of Nine, by Star Trek: Picard time, be a mercenary?
Within Star Trek: Picard, there is Cristóbal Rios.
Rios, a former Starfleet commander, rose to the post of executive officer on the USS ibn Majid, a heavy cruiser, where he served under Captain Alonzo Vandermeer. His serial number was SC-850-705. (PIC: "Broken Pieces")
By 2399, Rios had become acquainted with Raffi Musiker, and had assumed command of the starship La Sirena, serving with his fully utilized Emergency Hologram basic installation, each of which was programmed in his own image .... That year, he was recruited by retired Admiral Jean-Luc Picard to aid him in his search for Bruce Maddox. (PIC: "The End is the Beginning").
Assuming you allow for the secondary canon of the books:
- Barb: Barbara, or Barb was a Human woman of the 24th century, a former Starfleet cadet, and a mercenary in the Silver Paladin's band.
- Darryl Adin: Darryl Adin, also known as Adrian Dareau and the Silver Paladin, and simply "Dare" to his friends, was a Human man of the 24th century and a former Starfleet security officer turned mercenary freedom-fighter.
As regards Seven-of-Nine, her only occupations seem to have been Starfleet and the Fenris Rangers, who seem to be acting without seeking payment, sort of like the Maquis, who had former-Starfleet Chakotay in their ranks.