Desk cycle tightening already tight hip flexors?

[GlassFrame]

I've suffered from various injuries for years which finally culminated in a bad back injury (L5 fracture) last year. My physical therapist at the time agreed with me that it was most likely due to postural muscle imbalances in my hips/glutes, which is likely due to whatever activity I do inevitably ending up sandwiched between long bouts of sitting.

Problem is, I'm looking at switching to working from home, which will greatly increase the already high amount of sitting in my day. So I'm looking into getting a standing desk and a treadmill to go with it, but I don't think that will be viable to the work that I do (3D modelling).

But my concern is that using a desk cycle instead will only tighten my hip flexors more and exacerbate rather than improve the problems I already have. I may eventually get both a cycle and a treadmill, and alternate between the four (sit-stand-cycle-treadmill), or is the tightening effect of the cycle a big enough concern that I should ditch it entirely?

Also, what's drawing me to the cycle instead of treadmill is that I'll inevitably lose proper posture when I'm focused on my work if I don't have a backrest, and a backrest will help me to keep proper positioning even when I'm not paying attention (albeit not perfectly). But that will still be inactivating my core+glutes and tightening my hip flexors. I'd like to know your thoughts on that.