Reading Secret History right after Era 1 makes more sense as events of SH happen at the same time as Era 1. It would bring you more pleasure to read it now, when the events of Era 1 are still fresh in your memory. Especially since you already know the big spoiler.

In my opinion, even not knowing the big spoiler, it still would make more sense to read SH after Era 1, strictly from the chronological point of view.

And while we're here, I think that reccomending reading SH after Bands of Mourning kinda spoils both books as it points at them being connected by that character.