Tips for running The Wild Beyond the Witchlight starting at Level 5

Hello all!

I'm about to begin the campaignThe Wild Beyond the Witchlight with my players, but because we're tagging it on to our current story arch, the PCs will be starting at level 5 instead of level 1. What are some mechanical tips or adjustments I can make to ensure this module still presents a challenge starting at Tier 2? 

I plan to bump up DCs for activities by 3, 4, maybe 5 points during their time in the carnival, but what about creatures and monsters? What are some small things I can add to increase their challenge ratings? Also, I've read that the BBEGs, i.e. the hags, and some of the other monsters later on in the campaign are not all that strong. Any specific tips/advice for making the Feywild encounters more challenging?

Thanks in advance.