Just in case people are still having problems, I managed to solve mine - after doing everything mentioned in this post, I finally found this article: Android Marshmallow problems and how to fix them - AndroidPIT
I updated all my apps, then cleared app cache data.
Some old data from some apps was causing the issue. After dealing with this for a week, it is a breath of fresh air to have a strong wifi signal again!
Probably not the solution for everybody, but it worked for me!