Another tip is to force yourself to read in half your normal speed. You’re not going anywhere, and reading is not a competitive sport. Personally this gives me a ethereal and meditative sensation, making literature more impactful.
Like you mentioned this is the opposite of “online reading”, where the point of reading often is to consume the content and move on.
Another thing that helps reading more slowly is reading out aloud. Imagine recording an audio book. I do that sometimes. It's somewhat soothing and also improves my pronounciation, which may seem like an irrelevant aspect but while my english is quite strong and I always read, listen to and watch english content in its original language, it's simply not my native language. Having the chance to read aloud/talk in english is a welcome and somewhat educational change.
I’m not going to pretend that it has somehow rocked my world to spend an hour or so a day reading novels, but... it has forced me to slooooooooow doooooown. A novel comes at you relatively slow compared to reading 15 news articles, Reddit threads, my twitter feed, et al.
It has been a peaceful experience to have a slowform entertainment medium take a chunk of my day.
And I always feel this is worth repeating, but after ~25% of a book if you’re not enjoying it, put it back on the shelf and try something else. We’re (most of us) not in high school anymore, nobody is forcing us to finish a novel. Read something because you want to.
Last note: if you’re having trouble staying focused on a book, put your phone in another room while you read. Game changer wrt the temptation of checking notifications.