I would love to step back into my old role. How do I sell this to
If you have a decent rapport with your boss, then you simply have an honest discussion.
You talk about the stress of the job, and how you no longer wish to deal with it. You don't need to get into the reasons so much (now that I am financially stable, I don't need to put up with the stress) unless you are specifically asked why.
You point out your value to the company, and how you will retain much of that value by moving to an individual contributor role. You hint that this is important enough to you that you would consider leaving, rather than remaining in the management role.
And of course you indicate that you realize you may have to forfeit some salary and benefits with this move, but that it will be worthwhile for you.
Some employers are flexible enough that they are willing to demote you and replace you. I worked at one such company, where I was brought in to replace the former manager when she concluded that she was over her head in the management role and wanted to go back to being an individual contributor. It was awkward for a few months, but worked out okay. Eventually, she wanted to go part-time, and later on left the company.
Some employers aren't as flexible, or don't want to deal with the awkwardness that will inevitably arise. Some folks who talk about taking a demotion to move to an easier job will still want to maintain some control and influence that other individual contributors wouldn't have. Some employers worry about that potential conflict. In your case you want to assure management that you won't be like that.
The only way to know is to talk with your management.
You might also consider if a transfer to a different department within your company would be a viable alternate solution. Sometimes, moving away from your current team eases some of the awkwardness.