I think i read once about there being a novel where it was just that, the main character reincarnating inside the mind of the villainess of an Otome game, but said character was still around and the MC could only give advice or talk to her in her mind with no one else being able to hear her, and that actually sounds cool...but i forgot the name, anyone knows what it is?
Sounds interesting, why risk yourself and reincarnate when you can just be the MC's old grandpa spirit, except in otome ofc.
There's a manga very similar to that, no villainess I don't think but otherwise very similar. Junai Sensation I think it was called. Unfortunately I don't know of any novels like it though.