The fan theories about where the series could go next are fun, but for now, the official series order is a closed loop. Two books that complement each other perfectly. Trying to insert other stories in between would disrupt the very specific emotional journey.
Okay, hot take: you could technically read 'It Starts With Us' alone and follow the plot of a couple getting together, but you'd be missing the entire point. The point isn't the 'what', it's the 'why'.
The first book's intense exploration of cyclical violence and difficult choices is what gives the second book's domestic bliss its meaning. Without that background, the sequel reads like a fairly generic romance. The power is in the contrast, so please do yourself a favor and read them in order. It makes the hopeful ending of the duology actually feel earned.