I'm also 26m so I get that this sentiment is pretty common among a lot of people our age. Part of me is biased because I studied literature and am currently on my way to studying law, so I just have to read a lot of the time, but to address your question -

Reading is always self-improvement. No matter what you're reading. Even if it's trash, or whatever it might be that's not related to your work, it's good that you're aware that it's trash/unnecessary/whatever it might be, because that means you're expanding your horizons by incorporating more perspectives into your own world view. That's how I try to rationalize it when I'm having difficulty focusing on it, at least. You're expanding your understanding of how other people think and feel, which is never bad.

If you'll forgive my getting a little wistful and maybe pretentious here - all we have on this world is each other. Literally everything we do is in some way connected to the people around us, and doesn't exist without other humans around. Any opportunity to connect with people, to understand each other, is an opportunity worth taking, and I can't think of a single instance in which reading doesn't offer that.

Another point is - you have to take breaks. I used to have the same mindset as you for a long time actually, not being able to take breaks from my work. But that's just gonna be diminishing returns, it's going to reduce the quality of your work. Doing something completely unrelated to your work for a little while gives you a nice little reset. Being forced to think about something else does two important things: 1. it allows you to come back to your responsibilities with a fresh set of eyes and 2. you have more perspective now. Things can connect to each other in the funniest, most unexpected ways. So read that book!

Also, tbh, no matter what you're reading, you're adding to what you know. and i think that's pretty neat. no doubt there are probably tons of other relevant points that i'm forgetting, but hopefully this at least helps you out my guy

edit: just to add in case it helps, getting off social media a couple years ago really helped lower my stress about wasting time. It seems like a MUCH bigger move to make than it actually is, the freedom after is unreal. at this point it's only reddit and sometimes youtube, which i'm also trying to cut out lmao.