I think I agree with the idea that it is better to turn the smartphone into the phone you want, but I think it might require a bit more work than just not installing TikTok :)

I doubt you'd see many ads in the iPhone Assistive Access mode, which can (sort of) turn an iPhone into a feature phone. You might also need to swap out a few Apple apps though. https://support.apple.com/en-sg/guide/assistive-access-iphon...