Just finished binging the whole thing, and I'm still reeling! For 'Divine Rivals' and the sequel, 'Ruthless Vows', the order is straightforward: read them as published. 'Divine Rivals' first, obviously. That's where you meet Iris and Roman and get hooked on their whole rivals-to-lovers war correspondence vibe. It ends on a massive cliffhanger, so you'll be desperate to pick up 'Ruthless Vows' immediately. I wouldn't recommend reading anything about the second book's plot first, because the mystery of where the characters end up is half the tension.
Honestly, I've seen some people suggest reading the bonus chapters or the author's online snippets in between, but I think that just muddies the water. Stick to the core novels. The pacing between the two books is pretty deliberate, and jumping around might spoil the emotional build. The order is simple, but the journey it takes you on is anything but.