There is so much activity in these journals! This has definitely become a journal board. I looked and looked and coudn't find your other sumo vid, Hits, before this one. If you could repost that or link your youtube page that would help. I think your sumos have, well, let me see the old vid first
Besides the activation drills you could also try split squat isometric holds. Start out with the side your having trouble with. The holds are exactly what they sound like. You hold at the bottom position and while you're doing that you can concentrate on engaging the glutes and hams. BTW, you need to have a certain amount of muscular tension to really help you engage and get power out of the big movers. If you're dong the split squats in a way that your flexiblility just allows you to drop down into it very relaxed that is not going to work so well. Remember if you want more hams and glutes you want to widen the stance width. And push more off the heel.
I see nothing wrong with your form on those.