Apologies for the wall of text. I wanted to give a comprehensive answer.
I tried using a Nokia dumb phone at one point. At another point I used the LightPhone2 as well. I honestly found usability very difficult. The Nokia was arguably more functional. Though it had a web browser (not sure if you are hoping to avoid that). However it was limited to 3G, which slowed things down. Calling was fine, on the Nokia and Lightphone. Texting on the LightPhone and Nokia is a pain. I didn’t like that I had to go through the LightPhone website to upload music I owned to it. Also, it had a very limited amount of storage. I didn’t like that I couldn’t stream music. The Nokia, I had also did not have music streaming functionality. However, it did have a micro SD card slot, which I liked. The camera quality was bad. Also at the time, I needed a way to scan QR codes, which neither phone had the ability to do. Both did not have any GPS functionality (at least at the time), which made me rely on a GPS in my car, which was not great overall. Google maps is a lot better. I hate Google, but use Google maps, as it is what I’m comfortable using.
Also, I had concerns on the security of these devices. I care a lot about digital privacy and security. The privacy policy of the LightPhone was not something I was comfortable with. Additionallly, I avoid text message, and opt for signal, as text and phone calls are very insecure and are far from private. Also, Google own KaiOS, which is what the Nokia phones use. So privileged admin access is given to Google to collect whatever telemetry that they enforce in the privacy policy of the Nokia dumb phones. I now use GrapheneOS on a Pixel phone.
What I will say is it is definitely harder to be distracted by your digital devices, when using these phones. I found it very hard to not have a smartphone for the time I tried both of these options. I didn’t have the will to stick it out. I also noticed, my internet addiction moved to my laptop. So it didn’t actually solve the issue. I understand the struggle of internet addiction, and how consuming it can feel at times. You may find a dumb phone is a viable option for you personally. I would suggest exploring other options before the LightPhone, as it is quite expensive, and if you don’t like it, you may feel as if you spent a lot for a phone that feels clunky. The Nokia’s may be an option depending on what you’re hoping to get out of it.
I see a lot of people on YouTube, finding it has had a positive impact on their lives. At least in terms of distractibility, which appears to be your goal. If you are willing to stick it out, it may work quite well for you. But you will need to make some trade-offs and be willing to stick with it.
On the note of blocking apps, there may be options that are reliable. Lock me out works quite well on Android. But you have to pay for the full functionality, and it is proprietary software. I use it anyway, because it works very well for me. It is a trade-off I am willing to make. On iPhone, I’m pretty sure you can use the default screen time rules, and get a good friend or family member to set a password for you. It is very difficult to bypass both of these options, if setup correctly.
Best of luck with your decision, either way. Feel free to ask any additional questions, if you want to know more.