What is the expected level of commitment for postdoc positions?

Every position is different. Every PI is different. Every field is different. In math you can probably set your schedule to suit yourself. In some lab sciences with demanding PIs it isn't so easy. But a few points to consider:

First it is fair to explore your working conditions before you start, though it may be a bit risky. If you can manage a visit, you may be able to get a sense of it from other post docs at the institution. But you can also just ask for expectations. You aren't required to damage your health, but some PIs don't know where to draw the line.

Next, you may find that the burnout you experienced and your need for a reduced schedule doesn't continue into the future and it might just go away. Lots of people have such a period, some in the middle of their studies and some shortly after. But a change of scenery, literally, might have a positive effect.

Don't worry about the future too much. Your situation, health, and attitudes may change. But a positive post doc experience could go a long way to helping. Many professors work long hours, but it is because they truly love to do the work. It ceases to be a job and becomes an avocation. What could be better than to be paid to think?

If you take time off, make sure you can explain it as something other than recovery. Have a flexible plan for your return if you decide to do that, and find something to do that keeps you from getting rusty. If you come back, you want to seem enthusiastic about it to anyone who reviews your CV and SoP.

I would say that the expected level of commitment of a postdoc is extremely high.

The expected level of a PhD student is very high. Being a postdoc brings you even closer to the "top" in the pyramid of academic life, hence it is extremely high.

Notice that this has not much to do with your PI or your postdoc host. It is you who should expect to be extremely committed in order to be competitive. Your host may be already established, if you are not extremely committed he/she may be somewhat disappointed, but it is you who is going to pay the price and need to end your academic career.

First, at the risk of stating the obvious, a postdoc position is a standard work contract. It happens to involve research work in academia, and it is true that there is often (not always) a culture of competitiveness and high expectations associated to it. But like any work contract you are only required to do the job that you are officially paid for, and pressure by a PI to do more than that is moral harassment, plain and simple (and it's illegal in most countries). Sure, it's quite common for postdocs to put in more hours than required, and some PIs might even expect that, but this is definitely up to you and nobody else.

So no, it's definitely not unfair or dishonest to take a postdoc position without intending to work more than what the contract says. Same thing for the intent to stay in academia: it's a temporary contract, not a marriage! If the institution wants their researchers to stay, they should at least offer permanent contracts. Additionally the vast majority of people who do a postdoc don't end up with a permanent position in academia, and many of them know from the start that they want a job in industry, they just want the postdoc on their resume.

Certainly there are places where the high level of competitiveness would make it hard for you to keep a healthy work-life balance. But this is not the case everywhere, there are very decent institutions and PIs who are perfectly ok with postdocs doing their research work like normal human beings. I've been a postdoc for 14 years, in the same institution for the last 10 years and apparently my work, while not especially impressive, is judged good enough for my contract to be renewed every time. I've been on a 75% part-time for the last three years by choice, simply because I prefer to have some time to myself rather than a full-time salary. So yes, it is possible to be a postdoc and keep your sanity too ;)