New guy here, but as a 47 Y/O professional firefighter with god knows how many injuries under my belt with 9 years left to work, I can only recommend a few things. Squat to depth with weight, as much weight as you can while maintaining good form. AFAIK, unless genetic, or injury related, most pelvic/hip flexibility issues can be attributed to either hamstring or quadricep inflexibility and strength imbalance. This creates the pelvic imbalance that generates low back pain and pelvic pain. Stretching is all well and good, but it doesn't correct the strength imbalance or weakness that created the issue to begin with. We won two world wars without stretching. I've never once stretched before jumping off the engine or truck and going to work with 65 or 70 pounds of gear on.
Don't forget front squats, dead lift, RDL, and as your flexibility increases over time, cleans. In my early 40's, I was noticing significantly more back pain, and hip/pelvic pain. I was squatting heavy, but not to full depth, often only to 3/4 depth. I dropped weight, and for almost a year, did nothing but ass to grass squats. It has changed my life. I now split my squat days with low bar back squats to depth (crease of hips below top of knees) and ass to grass front squats. That with including RDL in my deadlift routine had pretty much eliminated any knee or back pain I had. This is just my personal experience, but I've seen it work with quite a few of the other older Firefighters at work.