My two cents would be the best approach might include music source separation, maybe in combination with traditional upmix tools.
My understanding is that source separation has largely replaced the spectral editing tools that people used for this in the past, which were VERY labor intensive.
That would get you Vocals, Bass, Drums, and "Other" (and depending on the tool, maybe Piano as well), that could be placed / mixed in different channels.
Stereo imagers could be used at that point, and the results upmixed into a 5.1, giving you some of the above in 5.1 to mix in.
That's what I've been doing in my stereo or 4.0/5.1 to 7.1.4 up/remixs.
Re: mono to stereo imagers, I've never tried (or have forgotten) the Ozone imager but in the past these all worked with offset comb filters to create the left and right from mono. It gives the sense of stereo, but there also a "phase pressure" affect that can be ear fatiguing. Also you have to be careful with other mixing steps after.
I learned that the hard way using an Orban Stereo Synthesizer in the 80's, to remix some Jazz I recorded, for broadcast on the local public radio station (where I worked). It sounded great in stereo, but those listening in mono called to ask what happened to the vocals. Sure enough, listening in mono the vocals were almost non existent. My fault, and a good learning experience as a music engineer/producer.
But in general mono to stereo/remix is whole hobby in itself, with some very active people in it. I can ask my remixing friend for links to mono to stereo remixing communities if you are interested.