It's been some time since I've referred to the book, but I don't think you need to do "nothing" on the non-lifting days. You can do some cardio work on those days, just be cautious about allowing enough recovery time, i.e., don't do intense interval training the day after a big leg workout. Sounds like this may have been one of the reasons you haven't made progress in the last couple years.
IMO, stick to the programs in the book, unless you have enough experience in programming to make some educated decisions on that issue. However, that doesn't seem to be the case given the last 2 years.

Don't second guess the pros.
I did the whole book and made great progress in fitness and strength.