Glad you have it figured out!

I'd recommend checking in with a professional (GP, physiotherapist, sports masseuse) if you need to get that tightness worked through. You can try get at it yourself with something small and firm like a lacrosse or golf ball but it's pretty tricky to get at this muscle yourself given how deep it is. I've managed to relieve it on occasion lying on my back with a lacrosse ball wedged under. I move it around the area in a gentle circular motion for a little while, then bring the knee of the tight side to my chest, then extend it again.

Tight hamstrings and sleepy glutes exacerbate the issue, and I'd recommend a short routine of lower body static stretches (28-30s each) for your QLs, hams hips and so on before bed (and maybe some shorter, more active mobility exercises when you wake up too). kinda boring on its own but pretty great with a podcast on.

And as you've already found, staying mobile helps a great deal. if you work behind a desk, it could be worth looking into something adjustable so you can stand whenever you'd like.