If you're able to get 8 reps on all 5 sets you should be using more weight, working up to being able to get 8 reps again.
"using a rep rang of 5x6-8 and when I can get 8 or more reps I increase weight?" is double progression.
Read the first section in this article: https://www.T-Nation.com/training/6-ways-to-keep-getting-stronger
You understand double progression, just not as double progression. If that makes sense....
Anyway, to answer your question, any model for getting stronger on assistance work, works. Don't sweat the assistance. Focus on the main, compound lifts.