I had a vaginal birth with an epidural. The pushing phase lasted almost three hours with me instructed to hold my knees against my shoulders for that entire time.

I think that's the primary reason why it took 18 months to recover.

Yes, EIGHTEEN FRICKEN MONTHS.
how horrendous for you, that is exactly the position you must avoid if you have SPD. Without an epidural your body would have been screaming at you to stop but with one you rely on your caregivers to consider your SPD when advising positions and in too many cases they don't think about your hips.

If it is any consolation almost the exact same thing happened to my friend (except she ended up with a c-section after the 2 hours of pushing which damaged her pelvis). She went on to birth twice more, both times vaginally, and by avoiding the pushing on her back with an epidural she recovered much faster from the births with no post birth physio required.