This is the thread you’re after:
LOTS in there to learn and consider.
However my instinct is that yours is a combination of imperfect bike fit and insufficient core strength and flexibility (hence the lower back pain). Some squats and deadlifts (if you have weights), or core exercises that improve stability and balance, plus some stretching of the glutes and hamstrings, could go a long way.
Then making sure your cleats are giving you a firm, stable foundation. I find that placing the centre of the cleat 1-2cm behind the first metatarsal on the big toe (easy to find; it’s the bony protusion on the inside of your foot and you can feel it through the shoe) should give a decent starting point.
Saddle height also worth checking. Lots of different methods, but my general rule of thumb is: if your heel dips, raise it, if your toe dips, lower it. Filming yourself from the side can reveal a lot you might not be realising is happening as you pedal.