To me the whole thing seems really complicated, The information I've found seems very black and white
It's not. Nothing in lifting is usually black or white. That's why there are so many heated debates over minutiae. Learning what works for you, your joints, your age, your metabolism, your energy levels, etc. is the real black and white.
Lifting is about consistency, progressive overload, a reasonable diet, etc. Rep ranges don't matter as much as long term effort.
Most of the research shows that at 70%+, at the 5 rep mark all muscle fibers are recruited. What this tells us is that if we stick with a reasonably heavy weight and perform 5+ reps, we're going to build muscle if progressive overload is involved.
My advice is to push every set for as many reps as possible, stopping each set shy of failure. Stay within the 5-10 rep range for most compounds, 8-15ish for most isolations, and maybe a little higher for rows, leg presses, squats or any other exercise you feel works better for you with higher reps.
I work in the industry and have met or interviewed hundreds of top natural bodybuilders. Very few of them work many, if any, sets under 5 reps. Point being, they are not limited by not working over 85%.
Parting words of wisdom...too many people chase after magical combinations, when the real magic lies inside. Get strong on the basic exercises, don't miss workouts, eat plenty of food, and you will grow.