How do you set and reinforce boundaries with your employer?

After coming out of a recent bout of burnout, I'm realizing it would've been easier if I was able to push back against unreasonable requests on my time. I work for a small consultancy, so they often need new sets of skills and their clients often care only about time-to-market.

Come to think of it, this has been true of most companies I've worked at. They talk a good game about best practices, agile, tooling, training, whatever... but when push comes to shove they'd rather have 3 more stories than honor previous commitments to regular tech debt stories, training, or even 40h weeks. They often can't even "waterfall" 2w of work without changing scope or priorities mid-sprint, and get mad when you bring it up/try to improve the system.

I'm tired of the same promises, but have also enabled this behavior-- I don't feel like I can say no. At my next company I'd like to gain their trust, but also set expectations and boundaries up front so I can have a life outside of work. I'd also like to push back against overtime or using my own time to brush up on some tech that they need because they were unable to plan properly (without sounding like a know-it-all). I'd like a two-way street.

*How can I achieve this while still gaining trust?* Are there good questions to ask or flags to look for in interviews?

On a related note: I've heard govt jobs are a good place to have a 9-5 and a life outside of work, but worry about outdated tech draining my energy (I'd be happy to have the time for hobby projects, but can lack the energy if work is hours and hours of mind-numbing stuff).