Has anyone mentioned about trying to slide your saddle forward or back a bit? I find when riding flat pedals my foot is a bit more forward compared to clipless pedals and move my saddle a little forward to compensate.
Another thing that isn’t all equal, is things like pedal and shoe sole thickness, both can make a difference.
Lastly, as we age, we get less flexible and a lifetime of being seated starts to take affect, which cycling can then exacerbate. So, things like functional strength training, general stretching and use of a foam roller (massage) can really help loosen up and activate your glutes, hip flexors, quads, hamstrings and lower back. Try to to the above 2-3 times a week. Worked for me and I had awful pelvic tilt and lower back pain.