Oh man. I’ve been in a similar position. In my case, I didn’t take the supervisory position, but then I was also prepared to walk away from the job if things got too bad. How far from FIRE are you? Or do you have a decent FU fund?
It doesn’t sound like it’s worth the extra stress unless you can train your replacement before to take over the supervisory role instead of doing both simultaneously. And then maybe delegating another experienced individual in the group as the tour replacement’s mentor.
You could also just stay in your role, and if the new supervisor mucks t up, remind yourself that it’s not your problem. That’s what I do now. I refused the team lead role, and I just keep my head down and do whatever is in my job description. If the lead messes things up, I sympathize but don’t help. My take is, if you agreed to take the role, then actually take the role instead of expecting me to do your job for you.