Weird. I basically got it by doing Stella focus route on day 1. And then banging Julia hard in the morning, then talking to Stella for the next 2 time slots.
There was no Julia sex during squats, but Stella did the lick.
My first play through I got no spicy scenes at all (talked to Stella in all three slots) .. and I thought this cant be all there is (much like my original first day playthrough). So I used unren and looked through the images. Saw the Stella ones, saw the Julia ones, so did exactly what you mentioned first (Julia -> hard, Stella, Stella) ... didn't get it. Tried other combos, other dialogue .. and so on, and so on. I'm not using x52 or console so I didn't look at what the values were at the end of yoga. After 5 or 6 different runs through the free roam I gave up and looked at the script.
To be quite honest, I've never been a fan of this style VN (many flags, many combinations of flags). Hunting for the right choices to get scenes. Or sometimes just stumble onto them by chance. Sometimes this is done well enough in other games, that you can predict from your choices where it might lead. Or a gallery that gives you hints as to how you might unlock a scene.
But even the first dialogue choice in day 1, with no real context to the answers, can lock you out of content with Stella. IMO, if I just focus on Stella, I should get Stella content. Yea, its just how I like my VNs. I really wanted to play and enjoy this though, love the models for all three main female characters. Ill probably just unren future versions and look at the lovely renders