The best part is that once you have proven expertise and an impressive resume, you don't even have to sound like you're saying no anymore. After being a lackey for a long time, it feels wrong to say no because it makes you feel like an asshole, but the reality is that there's only so much time; there are only so many hours in a day, and you have only so many days left in this world, and you should expect to actually enjoy some amount of those remaining days. Plus you start to realize that your value far exceeds your compensation, otherwise a company whose sole existence is for the purpose of profit would be incapable of existing since there is no profit if the labor is paid what it is objectively worth. So you just pick your battles and tell people to fuck off when they overstep. It costs money to hire and train a replacement, so unless you're already highly compensated, you have the power to say no to egregious asks and you really should, or you set precedent that you'll say yes to that type of shit and they will continue pushing until they find the line where you finally say no. There is risk that they'll fire you and figure out later that it takes more than one new hire to do what you were already doing without considering the scope creep, but with a good resume and a healthy savings I enjoy playing chicken with a bad boss.

I've been with my current company for almost 3 years and I've only had to say no a couple of times. They're far from perfect, but they're good enough that I actually don't like to say no when I have to here. They pretty much always have reasonable asks. I'm 35 but I could actually see myself staying here until retirement unless they drastically change. I know for a fact that I could go elsewhere for similar pay and treatment pretty easily (because I've interviewed and received offers but turned them down because the pain of change wasn't worth something lateral), so I'm ready if they do pivot to fuck this environment up but I'd really rather stay.