Reasons why K-12 took so long was because the recording sessions began prior to her working on a film. K-12 was her first time writing a movie script, and once the original script was finished she got told that it would be too expensive and she had to spend time rewriting it etc
Keep in mind that alongside recording the MM3 album, she's apparently been writing the movie script since the beginning of this year. And she's more aware of what can/can't be afforded now. K-12 movie started filming in late 2018 and the editing had been finished by May 2019, so if she starts recording the MM3 film later this year/early next year, a 2022 release isn't impossible. It's not a Melanie release without some sort of delay but I have reasons to believe the album could actually come out next year.