Does Allah's statement in Ayat 66 of Surah Al-Anfal "الآن خفف الله عنكم و علم أن فيكم ضعفا" mean that Allah only knew at that moment that they were weak or if Allah had known it since the beginning of time, and what is the reason for this?
The meaning is not what you have written which is a superficial linguistic translation..
'Allah Knows every thing in the pats, present and future. The verse means that : Now, Allah Lightened you burden when it became clear for you that you have weakness.
Wassalam.