The Mind Illuminated is best thought of as a lab guide, not a novel. It’s best to read just up to the Stage you are at.
The reason for this recommendation is pedagogical. Each Stage introduces new terminology, in the form of bolded terms. Each of them, despite them being somewhat generic-sounding (e.g. “forgetting“) refers to a specific mental phenomena. At the point you first read a stage you will have no idea what the book is talking. As you do the exercises in the Stage, you’ll ground the terms (“Oh! that’s what that means!”) in your experience.
My general recommendation is to read up to the chapter you are in. Do the exercises. Somewhere in the middle of that read the chapter again – it will make a lot more sense. As you approach the end of a Stage, read the next one. When I finish a stage I tend recommend reading it again, just to solidify the ideas and terms. On each pass through the chapter, it’s like I’ve never read it before (!)
Reading far ahead will just confuse you.
You can also skip the Interludes. If they help you, great! But don’t feel they are necessary.
Reading the Stages, multiple times, is recommended.
To get started, I recommend reading up through Stage 2 and then starting the Stage 1 and Stage 2 practices. There’s a lot of words in the book, and you won’t know what’s important. Don’t worry.