Katey Sagal has been one of the lead characters on Futurama for its entire run. And during that run she was also one of the lead characters in Tucker, 8 Simple Rules for Dating My Teenage Daughter, The Search for the Funniest Mom in America, Sons of Anarchy, The Bastard Executioner, A to Z, and Rebel (although there was no overlap among these other series).
As said, it’s much easier these days with short seasons and long gaps between them. Ella Purnell was a lead in 3 shows that released last year (Fallout S1, Sweetpea S1, Arcane S2).
Bob Saget was hosting America’s Funniest Home Videos and Dave Coulier was hosting America’s Funniest People while they were both starring on Full House in the 90s.
Terry O’Quinn had a regular role on Millennium from 1996 to 1999, and did ten episodes of JAG between 1995 and 2002.
Her voice work on Bojack Horseman also overlapped with the final seasons of both of those shows.
If you include voice work, Frank Welker was essentially in every animated show in the 80s and 90s, and still working. Same for Peter Cullen, Jim Cummings, and Tress MacNeille.
Back in radio days, Dennis Day was a regular on the Jack Benny show and on the Dennis Day show at the same time
Colin Jost is both a member of the SNL cast and the host of Pop Culture Jeopardy?.
If you are going to go with prolific voice actors Phil Lamar has 580 acting credits. There is a lot of overlap there.
Jack Quaid has been starring on both The Boys and Star Trek: Lower Decks (plus he appeared in the latter’s crossover episode with Star Trek: Strange New Worlds) for a few seasons now.
In fact, the other night, Walton Goggins was in The White Lotus and then The Righteous Gemstones immediately thereafter.
I am not sure if Oliver Platt is considered to be “starring” but he plays prominant parts in Chicago Med and The Bear. I was quite surprised that he plays such different characters, the kindly understanding Dr. Charles and the slightly menacing Uncle Jimmy. To me it is sort of strange watching them both each week that (to me) he even looks slightly different in each role.
Also Rob Lowe, 901-Lonestar 2020 to 2025, Unstable 2023 to 2024, The Floor 2024 to 2025.
Currently Wendy Malick is on Night Court and Shrinking. I doubt Shrinking is heavy lifting. She’s in a supporting role and there is a lot of time between seasons.
John Goodman is appearing on The Righteous Gemstones and The Connors this week. Though as you said, both have short seasons.