It may be for you personally that is the case and I’m not arguing with your other points and logic on this statement for those purposes.
But actually, research shows reading + listening is a valuable activity and enhances vocab retention. (See p 18 of this guide, there is research to back up all of his statements in this guide). So for the purposes of vocab retention it can be recommended. While the two activities alone might be very challenging, the two activities together, by virtue of making it easier, can be a massive benefit to someone in making the immediate connection between what is seen and what is heard faster and therefore making the learning faster. It’s later when that is too easy for some material that separating the reading and listening to get the extra challenge you mention may be important.
It’s not an either or thing here, people can spend some portion of their time doing any number of activities separate or together. Since OP is talking about books they find challenging, I think this point about listening + reading is relevant to them.
Others mentioned combining reading + listening was helpful. My subjective experience aligned with this strongly as well
having diligently tracked my vocab growth scores with various strategies of reading in the last year, at the earliest stages I also noticed periods when I intensively did reading + listening on Satori - that grew my vocab faster than when I intensively read (no listening) elsewhere. Surely, part of that was that Satori is superior for time efficiency of lookups, but even then, all things being equal, I saw this effect several times and now that I’m reading post-Satori I notice a huge difference in my recall of words I learned with reading+ listening is far superior than words learned in other ways. I think the immediate connection helped a lot. Now, when I encounter those words, if I just pause to internally “listen” it’s like a sentence fragment I previously heard surfaces in my mind and I immediately get the sense of the word. These words that I learned with reading + listening are therefore cemented so much better in my mind than any others because they are wrapped up as a complete package that easily accesses meaning, context and pronunciation. I can only whole-heartedly recommend lots of reading + listening, especially at the beginning stages of reading before its totally fluent.
I don't think I did anything different than what you are already trying, to be honest with reading, then reading + listening and it worked great
The way I dealt with something too challenging to read + listen in one go, or to listen to all of it in one go is to break it down on a day by day basis. Find the passage length that I can do this routine every day: First read it intensively (with lookups), then listen + reading, then repeat. The next day, listen plus read again. If it’s too tough to follow at the native speaker pace: first read (with thinking time), then read + listen. Then do the next passage. At the beginning stage my passage length was about 200 characters, and for about a year I did this in ~400 character chunks on Satori. Eventually I became able to read longer and longer passages per day. Once it got too easy to repeat so much I either went for more challenging reading or I would listen to longer passages further away in time from when I read them.
Eventually, I learned to read more extensively (guessing words, not so many lookups), and then by extension to listening more extensively (without reading or stopping much to do lookups).
My strategy now is that daily engagement with the language is most important, and that is going to happen best if I’m interested. So I’ve let go of the efficiency mindset and I just read and listen for enjoyment, with the one condition that I do it in some form every day.
Your strategy of listening to the audiobooks while reading book to nail down those guessed words/unknown readings sounds great. Maybe just do that every day for six weeks and then try reading something from the start and see if you improved?
