i personally don't think there's any major co-relationship between FE and PE. They are totally different exams. Obviously, one is closed book and one is open book.
When i prepare the PE, i never look at any material from FE. The PE exam study is quite independent.
You don't have to spend more than a day to look back the FE stuff, i believe those basic knowledge are already embedded to your mind.
FE is more about theories, while PE is more applications oriented. (i took FE in 2012, PE in 2017, unless FE exam has been revised during these years)
FE is definitely much easier during college or right after school, because we were at the climax of Theories at that point.