Hi Losingtheplot,
As I said, I'm no expert, but please don't double the dose on the 'Sandrena days' as you will get even more of a dip on the days you are not on it.
My advice (again, I am no expert) is to take the Sandrena daily. It is how its designed to be taken, peri or post.
I'm assuming the implant is progesterone only? So your body is probably up for some more estrogen?
Sending love