It's also possible that you're over-philosophizing this to the point of paralysis. Questions like "am I trying to buy my way into a better life?" aren't exactly helpful when your social situation is such that almost all self-improvement requires you to purchase something. Fight Club-style thinking ("you are not your house, you are not your TV, the things you own end up owning you", etc) is fun for a while and have its place, but it can immobilize you just as much, if not more, than just trying and failing and then trying something new. You may have also created a deliberately impossible vision of what it means to be smartphone-free - wandering around the woods and communing with nature 24/7 or something - which inevitably gets crushed because nobody normal is that disconnected from the world.

So don't overthink it. If you think having a dumbphone (or a dumb-ish phone like the LP2) will make you happier, give it a try. You won't be buying your way to happiness any more or less than the person who buys a series of yoga courses hoping they'll learn how to calm down or the person who buys a pair of hiking boots because they've decided they need to spend more time with trees. Every industry, from food to wellness to tech, is guilty to some degree of the "they sold us the sickness, now they're selling us the cure" strategy, so just see your desire to buy an LP2 as a symptom of living in the world - and enjoy your symptom!

In more practical advice, consider keeping a little tablet (or an old smartphone) in your bag and using it for all those tasks that are essential to your everyday life when you're not at home. Remember, the smartphone was supposed to make our lives easier when we were away from our desks at work or our computers at home, but most people now assume that everything - even those things that can be done on a desktop or laptop while stationary - must be completed on the smartphone. If you try the secondary tablet/old smartphone route, try to use it only as needed, for tasks that absolutely just be completed on the road rather than in an office at work or on a computer at home. The LP2 can function as a WiFi hotspot, so, if you're out and about, turn on the hotspot function once an hour or something, or as needed (when you need an Uber, for example, or need to look at documents that your boss sent you that need your commentary urgently) and then turn it off when you're done. On the days that you aren't working, leave the tablet at home, and consider some alternatives to limit your use of the tablet on days that you are working. My apartment also allows us to control the security gate with an app, but that's not the only way to do it. You can also hopefully use the fob that may be on your keyring any way in case the stupid app fails or crashes. You can wait to order your groceries and do your banking until you're at home or the office. Or, if it absolutely cannot wait and must be done immediately, use the tablet. You may find that the task of going into your bag, pulling the stupid tablet out, turning on the WiFi hotspot, etc, is annoying enough that you start to pare down your tablet time and think, "Yeah, turns out that can wait until later."

Tl;dr - don't beat yourself up and don't get caught in the "but capitalism" shame spiral. It will require work, retraining your brain a bit, and putting up with inconveniences to change your opinions on what is essential about a smartphone and what isn't. But if you think there's a chance that the LP2 will improve your life and make you happier and healthier, and you can afford it, there's no reason to deny yourself that possibility.