G: Yeah — if “9 to 5” as Frank-N-Furter got nixed, I’d switch to songs where Frank’s theatrical arrogance/sexual menace/camp can be laid over the song rather than songs that merely fit Tim Curry’s voice.

A few that strike me as particularly good:

“I Put a Spell on You” — Screamin’ Jay Hawkins — probably my first choice. Frank barely even needs to reinterpret it. Seductive, threatening, funny, and plenty of room for physical acting.
“You’re the One That I Want” — Grease — done as Frank, particularly if you treat the other half of the duet as someone he’s relentlessly trying to recruit/corrupt, could be hilarious.
“I’m Too Sexy” — Right Said Fred — almost stupidly appropriate. The trick would be playing it completely sincerely as Frank rather than doing it as a joke.
“Sex Bomb” — Tom Jones — same general territory, but gives you more actual singing and swagger.
“The Bitch Is Back” — Elton John — excellent Frank attitude, and the gender mismatch actually makes it better.
“Bad to the Bone” — George Thorogood — slower and less flamboyant, but Frank treating this macho-boasting song as his personal manifesto could work extremely well.
“Hot Stuff” — Donna Summer — Frank can absolutely inhabit this without changing much except the characterization.
“Love Potion No. 9” — The Clovers/Searchers — mad-scientist Frank gives the entire song a second joke.
“Devil Woman” — Cliff Richard — reverse the implied gender and lean into Frank being the dangerous object of fascination.
“Hungry Like the Wolf” — Duran Duran — surprisingly good if you make Frank predatory rather than New-Wave cool.
“Addicted to Love” — Robert Palmer — Frank playing the diagnosis as an invitation rather than a warning.
“You Can Leave Your Hat On” — Randy Newman/Joe Cocker — extremely Frank, although almost too obvious.
“Maneater” — Hall & Oates — again, gender-flipping the narrator/object makes it nicely Frankish.
“I Touch Myself” — Divinyls — shameless enough for Frank and funny precisely because Frank would see absolutely nothing embarrassing about announcing it.
“Let’s Do It (Let’s Fall in Love)” — Cole Porter — wildcard, but potentially fantastic if you want sophisticated, louche Frank rather than rock Frank.

And there are two actual Rocky Horror fallbacks worth remembering. “I Can Make You a Man” is only about two minutes and is essentially Frank showing off, while “Sweet Transvestite” is the obvious full-scale character showcase; both have commercially released karaoke versions. citeturn0search0

I would not choose “I’m Going Home” tonight unless you specifically want the room to turn emotional. It's Frank's vulnerable song already, and given Curry's death this week—and the Broadway cast literally using “I'm Going Home” for its Curry tribute last night—it now carries a lot more memorial weight than it normally would. citeturn0news24turn0news26

For what I think you were trying to accomplish with “9 to 5”, my first five auditions would be I Put a Spell on You → The Bitch Is Back → I’m Too Sexy → Sex Bomb → Love Potion No. 9. Those give you five substantially different ways of doing Frank sings a song that was never supposed to belong to Frank.