Entrepreneurial Management – Employee Bad attitude but great work?

I have this employee who has a terrible attitude. Sometimes she’s great. Most of the time, she is just unpleasant. She even jokes she’s a “miserable person” or a “b*tch” (like it’s her character).

She was my assistant but said she doesn’t want to do that work anymore (quit). But I brought up the idea of moving her into a managerial role instead. She does an amazing job with her work. She knows the business inside and out. Is self-disciplined, strategic (I can trust her beyond tactics). So part of me wonders if she has been miserable because she was in a tactician role?

I think others see she is snappy/cynical but more as a sense of humor but I don’t think anyone else feels her attitude – it seems to be more towards me. Also, with her “quitting/in-flux” she has been less than cooperative with training the new hire. She gave me less than 2 weeks notice before her vacation which was very stressful. I also have a feeling of “you’re lucky to keep me” versus “I want to stay.” She asked for more than double her hourly rate (from $15 to $35) and wants no contact with customers (“I don’t want to talk to humans.”)

Do I just let her walk? And lose 1.5 years of teaching her everything? She’s by far the most sufficient employee I’ve had and again, I enjoy working with her when she’s pleasant.

Or am I just trying to hold onto her because I am afraid of losing her?

Or do I put the job out there and hire someone else?

C – Employee is quitting. I was planning on her running the business while on maternity leave (we’re trying for a baby so it’s a while away)
T – I don’t want to have to train someone new. It’ll take forever to get them to her level. She’s already trained and does a great job.
F – Conflicted
A – Avoiding pitching her the new role
R – No progress. Spinning in my own head.

Thanks so much!