Trying to transition easily to my own company

I recently left a position at a firm where I was the only project manager,(small business) because I knew the owner was looking to retire soon, and I never felt I was recognized for all my efforts. She had no patience working with sub contractors, and was not even listening anymore to clients needs. Honestly I was the one that communicated with the clients the last couple of years. I hear from employers and sub contractors that since I left things are a mess.
I am very disappointed with the new job I took. I am considering leaving, and hoping to spend more time building up my own side company which I've had for the last 6 years.
I emailed my former employer, telling her of my disappointment, and that I would be either looking for a new job, or taking on jobs as a consultant. She immediately emailed me back that she wanted to speak to me.
I'm hoping to have her as my main client, how should I approach this, and how should I charge? I'm concerned about not getting a steady paycheck, should I just go back as part time, and continue to do my own thing on the side?

This is difficult to advise, because I believe the success of this relation highly depends on the individuals. From what you describe it actually looks like the company is a mess and the owner has at least an interest to retire without leaving such a mess.

You have the options to ask for the same job as you had before, but this leaves you the same situation as before. You have no power at all and act like a fire brigade. It's not a position which makes many people happy.

You could ask for equity and tell her to take over once she retires. That would be my first idea. It's interesting if you can buy out the last shares from heirs, if she dies or doesn't want to deal with the company anymore. After all, without you the company is not running. It will probably die if you could become a competitor. If she doesn't have so much interest in running the company it would be a good opportunity to save her life work and give it into good hands.

You could work as freelancer there. In this case I would charge the standard rate - a "i love you" bonus. For example, PHP developers in Germany get around 65 € per hour. I would work for 60, because she is your main client. As Java dev you could make >75€, but I would do it for 65€, for the same reasons. Also 65€ is good in Germany and a proper outcome. The other rates are including money to aquire, which you don't need.

After all I would be open and talk honestly to her. What do you want (more recognition, building up your own thing, but also you still care on the company) and what does she want. Simply ask her.

If she is not open to any change and just wants you back in the same role with the same money as before, this is most likely not going to work. You'll feel the same frustration as before. I would not do that. But if she listens to what you say and you have the feeling she really wants a change, why not trying. This might be a good opportunity for something better.