Sorry to hear about the burnout. It seems that the job, the manager and the understaffing are all going to be the same as they were? And I bet both the hospital and the manager think that "a gradual phased return" means that they expect you to go back to full-time work at some point, and probably that they will be doing everything they can to turn a gradual phased return into an accelerated return.
Once a person has been subjected to stress leading to burnout that the likelihood of stress and burnout returning is greater, that it happens sooner at lower levels of pressure and that the health consequences are more severe and long-lasting. Alongside my day job I was a trades union official in the public sector: I saw this happen to colleagues. That the courts recognise this and make provision for compensation for people who have not been protected by their workplace after a stress related work injury is still inadequate recompense for the damage it causes.
For you to go back to the same job, manager and understaffing is sending me danger signals. I think that as a minimum one or more of those needs to change before you go back, otherwise every day you walk into the hospital will be a day when the pressures that led to your burnout will be pressing down on you again, and any limits that you try to set in your own mind will have everything else - the understaffing, the attitude of your manager, your own desire to do a good job and help your patients to the best of your ability - undermining them every minute of every working day.
Sometimes as public sector workers it is hard to be open to other possibilities. But my advice would be that you should look at every other possibility open to you rather than going back to your old workplace. Another hospital, including a private one? Clinic work? Retraining in a different speciality? Teaching? Being FI gives you options. And if you have a partner then the future security of your children is down to them as well, it doesn't fall all on your shoulders, and if you are not working that frees them up to improve their earning power and take care of those potential future costs that worry you.
If you do stop work altogether, just remember to keep your national insurance contributions going so that you get the full 35 years of contributions. The pension might not be much but I suspect that if charges do start to be made for things like health care then people with a full record of NI contributions will be the last to be charged and be charged the least.