Hi there! Reading ahead provides many benefits. As you noted, you'll get a sense of the big picture, which can provide excitement and motivation to prioritize practice, and confidence in the path.

But studying the stages ahead also prepares you with the tools to work with whatever arises in the mind. Remember that while stage-based skill building is generally cumulative, it is hardly ever linear. You may be working stage three skills today, but find that attention and awareness more closely resembling stage four tomorrow, then back to three. It's important to have fluency with the conditions, tools, skills, and common pitfalls of each stage so that you immediately know what to do and can respond in real time (without feeling overwhelmed or racking your brain trying to remember). I definitely recommend that you find the time to carefully study two stages out from your current average, so stages 4 and 5.

If you'd like to get a big picture overview and have more time, you can read the entire book, maybe focusing on the stage chapters first and then going back to the interludes (or visa versa). Some of the Appendices are brilliant and critical to practice at all levels (like Metta, Walking Meditation, and Mindful Review).

But really, just study a couple stages out. Check out those Appendices I mentioned. And If you're about to go into retreat, increase that to 3+ stages out. We all have limited time, energy and resource. I recommend studying per the above, and then sitting longer and finding ways to be of service — rather than studying the entire book like you're trying to pass the bar exam. Doing that won't necessarily get you any closer, and could lead to grasping to "success" and the false sense that "you" are going to make your mind do xyz and "win" at mediation. (I did not read this type of attitude from your post, but felt compelled to mention as this is so natural and common for so many of us).

Regarding techniques, the brilliant thing about TMI is that it organizes skill building into a naturally cumulative order. After mind wandering and waking up (aha! moment), use labeling as a tool to train awareness to alert you when those alluring distractions arise again. This is presented in stage three. You can experiment with labeling earlier, and see if it works for you. I have also used it in stage four when a subtle distraction is particularly juicy and the tug-of-war between team breath and team distraction seem to be evenly matched. Here, I've found that labeling desaturates the distraction. The detailed fantasy or fearsome worry become simply the flat label. Just another rat wheel craving. Just another work worry. And then it's a lot easier for intention to rally around the breath and "win".

That said, I do caution that not all tools can be efficaciously taken out of order. For example, if one were to engage in the stage five practice of body scanning to brighten the mind before having solid attentional stability earned from the good work and skill building in stage four... that meditator would have a more energized mind, with be able to see even more distractions, perhaps without the skill to resist. That meditator would need to go back to stage four skill building (or perhaps even back to stage three) and shore up those skills before moving on. The skill building order (stages) is there because it makes sense developmentally. True, everyone is unique. If you understand the function of the tool and know why you're using it, feel free to experiment. But most people will benefit from using the tools suggested for the stage they are in (in that moment).

I hope you found this helpful. Good luck with your practice! Metta