I’m admittedly rather new to copyright/trademark law myself, so I think you’re better off asking those at the PDSH website, among others. For example, on https://pdsh.fandom.com/wiki/FAQ#Doe..._characters.3F, it’s stated:
“Trademark and Questionable Status?:
“Doesn't DC Comics own all of the Charlton, Quality, and Fawcett's comic characters?:
No. While DC comics bought all three of these companies, the majority of their comics were not renewed or had improper copyright notices. For example, Whiz Comics #2 was not renewed meaning all of the characters that first appeared in this issue are in the public domain such as Golden Arrow, Spy Smasher, Ibis the Invincible, and even Captain Marvel.
However, first thing to know is DC DOES own the rights to books where the copyrights were renewed such as Marvel Family #1, meaning DC owns the copyright to Black Adam (whose first appearance is in that issue). Second, DC also owns the rights to their versions of the Charlton, Quality, and Fawcett characters because they are derivative works. Finally, even if DC doesn't own a character, they probably own the trademark to any of the prominent character's names such as Blue Beetle, Plastic Man, etc. This doesn't mean you can't use the character, though, it just means you need to call them by a different name or only use the name in the interior pages of the book so as to avoid anyone confusing your version of the character for DC's.”
and
“How Do I Use These Characters?:
Any way you want! Download copies of their adventures and print them for sale! Write and draw new adventures for the Green Lama, make a movie about the Blue Beetle, write a novel starring Yank & Doodle! Use their images in advertising, on posters, for greeting cards, etc! Make balloons in their image or start a line of action figures that no child will recognize! Your NEW product is YOURS and no one can use it without your permission. Just make sure you DON’T copy someone else that’s using the same character, because THEIR new product is THEIRS!“