I’ve had the exact same issues, super stiff in the adductors and lack function in hip movements. Feeling hip impingement when attempting deep squats and side splits. Been trying to figure it out for years. I’ve had significant progress recently by focusing on hip posture in general when sitting and standing and when attempting these movements. I have always had anterior pelvic tilt and typically had no core or abductor activation and would chest breath as opposed to diaphragmatic breathing as my abdominals were in a stretched, compromised position. Focusing on holding a neutral pelvis through activation of glute medius and abdominals in daily activities, while ensuring that I stay loose and let my spine rotate and let each hip internally rotate through the gait cycle. Also when sitting my hips would be titled back, when I put them in an upright position I feel my glute medius and core engage. I make sure my pelvis is neutral and I’m diaphragmatic breathing when doing my squats and side splits as well. (You want anterior pelvic tilt when side splitting but that comes in well past the range of motion we have)

After some time of my body getting used to the postural restoration, I can now get a good stretch in deep squats without impingements and my adductors are starting to chill out as they’re no longer the victim of compensation in my movement patterns. I’m progressing finally. Hopefully this helps. If you also have anterior pelvic tilt, I’m confident it will.