There are two ways of completing the syllabus -
Just doing the syllabus and then levelling up
Levelling up along the way
First Method
Many students prefer this but it is not suggested to follow
Syllabus should be completed till October-November and our goal is not merely completing the syllabus.
We have to now do level-up ,means all the chapters you have done should convert to score for eq if you have covered 40 chapters out of 100 are you able to score 35-40% in JEE Advance test if you are not able to score firstly you have to focus on levelling up the chapter not running behind syllabus completion
The extra efforts you will be putting in levelling up the chapter will bring more score.
Disadvantage
1)It will become difficult to level up as gradually the syllabus will increase.
2)It will be difficult to align the thinking of chapter towards levelling up.
Method -2
The best time for levelling up the chapters is NOW . Complete your chapter and then pratice enough questions and hence levelling up on the go.
if you have want to clear backlogs as well you can refer this How to Cover Backlogs of Class 11th in Class 12th?