All right, Wrightline. This is one of those questions that would be impossible for me to provide a complete answer for without trawling with a fine-toothed comb through thousands of comics. After all, private investigators are part of what gave Detective Comics its name in 1937. But I can name a few who may have missed your eye.
You mentioned Slam Bradley, but there’s also Samuel “Shorty” Morgan, Bradley’s young sidekick who debuted right alongside him in Detective Comics #1. We find out that Morgan was killed in issue #572, 50 years later.
One of the earliest Quality Comics characters was private detective Hugh Hazzard, who debuted in 1939’s Smash Comics #1 as the operator of Bozo the Iron Man. Hazzard himself has never appeared in a DC comic since they acquired the Quality stable, but his Bozo robot has made a few cameos. A Quality contemporary of Hazzard’s, “Dick Manners, Super Sleuth,” debuted in Feature Funnies #17. The next year, Quality introduced married private detectives Jack and Jill Doe in 1940’s Hit Comics #1. Like Hazzard and Manners, DC may own the rights, but they haven’t appeared in a comic since their acquisition.
Larry Lance, love interest and eventual husband to the Golden Age Black Canary, was introduced as a private investigator in 1947’s Flash Comics #92. Later comics would establish that before entering private practice, Larry was a detective for the GCPD. His partner, Jack Lynch, eventually became a private investigator as well, and meets the new Black Canary in 1993’s Black Canary #12.
While we’re on the subject of Golden Age support characters, I’d be remiss not to mention Hiram “Stretch” Skinner, the lanky private detective and sidekick to Wildcat who debuted in 1942’s Sensation Comics #4.
One curious case I’d like to point out is 1957’s Batman #111, where we meet Bruce’s uncle Bruce N. Wayne, after whom Bruce himself was named. He’s a private detective from the west coast, and manages to deduce Bruce’s identity as Batman until the younger Bruce conspires to prove him wrong. After that issue, Uncle Bruce is never seen again.
Of particular significance is Tim Trench, who was introduced during Denny O’Neil’s run on Wonder Woman in 1968’s Wonder Woman #180. He’s Wonder Woman’s closest partner during this period which finds Diana without her powers, pursuing the villainous Doctor Cyber for murdering Trench’s partner.
Charlton Comics character Sarge Steel may be a government agent today, but he began in 1964’s Sarge Steel #1 as a private detective before he was recruited by the CIA.
You may be surprised to know that Jason Bard wasn’t the only Gotham PI in Barbara Gordon’s love life. Babs was briefly involved with another detective named Mark Hanner, introduced in 1969’s Detective Comics #384. Hanner is a diabetic, and it’s Batgirl to the rescue when he’s kidnapped and separated from his insulin.
There’s also three of the Courtney quadruplets introduced in 1976’s Detective Comics #460, Michael, Robert, and Jerome. Their fourth brother, Carl Courtney, operates as the Batman villain Captain Stingaree.
1978’s The Flash #260 introduces us to San Francisco-based private eye Philip Russell, in constant pursuit of a villain known as The Viper. Little does Russell know, thanks to a split personality disorder, that he and The Viper are one and the same.
Wyoming-based private investigator Harold Halston was killed by Vandal Savage in 1987’s The Flash #1, setting the scene for Wally West’s first case as the Scarlet Speedster.
An original television character, Megan Lockhart, was introduced as a recurring PI in the 1990 The Flash television series, who is kidnapped at one point by The Trickster and coerced into becoming his sidekick.
The politically radical left-wing, Boston-based private detective Micah Rains is a staple of William Messner-Loebs’ run on Wonder Woman, beginning with 1993’s Wonder Woman #73.
James Robinson introduced us to private detective William Von Hammer in 2009’s Superman #689, reviving the character for his 2011 Starman spinoff The Shade. He’s implied to be a descendant of Lieutenant Von Hammer, the Enemy Ace, but that relationship has never been canonically confirmed.
In 2016, the classic DC comedy strip characters Sugar and Spike were reinvented as private investigators for a marvelous run of features in the Legends of Tomorrow anthology, as the sleuths that the superheroes go to when they need an embarrassing case solved with discretion.
One of the most recent private detectives established in DC Comics is The Silver Ghost, who appeared in a Detective Comics #1027 story written by Grant Morrison. His true identity is unknown.
And last but certainly not least, there’s Sherlock Holmes, a real person in the DC Universe, as seen in Detective Comics #572.
Or do so under the authority of a police badge. Professionals such as Jonny Double, Slam Bradley, Jason Bard and the aforementioned Nathaniel Dusk.