Hey, do you actually have any back problems in the first place? Do you actually have painful back or shoulders? Because you're saying you want to prevent back pain, I'm not sure that you're actually already have this issue, or if you're just worried you will.
I do sit on my fat ass all day pretty much since the pandemic started. The only time I experienced back pains, that surprised me with their persistance and intensity was when I was briefly coming to the office at my new job at the very start (they let me work from home after a month of internship). I learned that the primary reason why my back hurt there is that all chairs and desks there are adjusted for comfort of an average person, and I'm basically a hobbit that is significantly shorter than average and I was way too anxious to come to the office and start re-adjusting my chair (with my luck I'd probably break it). I was also sitting in very stiff and somewhat unnatural pose and didn't take enough breaks - because of anxiety about being surrounded by people. As soon as I went home and switched back to using furniture that I adjusted for my 5 ft,. tall body, back pain went away almost entirely and only bothered me a few times when I would sit for really long stretches of time of 16 hours + without taking appropriate breaks. Getting up every hour or so for a bathroom break, to make a cup of tea, or to go grab a snack and just walking around your apartment for a bit really helps, and if you can have at least one break a day that is longer than that when you can go outside and idk, hit the nearby grocery store (I go to a coffemat that is about 500 m. away from my home), that's even better.
If you do experience back pains that are not alleviated by just taking more breaks and making sure your furniture is comfortably adjusted for your height, you might actually have some underlying condition and I strongly suggest seeing a doctor about it, because excercise is great for you and all, but with back issues you've got to be really careful about it or you'll be running a risk of making your situation worse.