Honestly, the idea of a 'full read with no interruptions' is more about finding the right copy than a specific place. If you own the digital file, any decent e-reader app in airplane mode is your sanctuary. I always download the EPUB from a store like Kobo when they have a sale, toss it into Moon+ Reader on my tablet, and shut off the wifi. Zero ads, zero 'next chapter' prompts, just the text. Public domain classics from Project Gutenberg are perfect for this too.
The subscription model on something like Kindle Unlimited gets close, but it's not flawless—you're still in their ecosystem, and sometimes the app nudges you to rate or shop. For true, walled-garden immersion, buying the file outright and using a barebones reader is the only method I trust. It feels like having a private library in your hand, no gates, no guards.