Lower your seat first. Might even need to move cleats slightly away from the front. Suspect u are tip toeing when cycling, ie heels up. Seems too much usage on your hamstrings and perhaps also hips rocking left right since lower back is sore. But after u change crank length, need to readjust seat height. Your problem now is likely over stretching and straining your hamstrings. Do u once in a while (esp in the middle of sleep) having cramp on the hamstrings? If yes, it is quite severe over working them.
Any particular reason why u adjust fore aft, and do u know what u are actually trying to achieve? Or just trying luck only?
For saddle fore aft, this is done after i have set the saddle height properly. What i did when i got the new road bike is to get the saddle height calculator and punch in the inseam height. Made a mark with a scotch take + marker pen and noted the height so that i can adjust easily in future. Actually that already felt pretty good initially already, but i saw another youtube video something about fine tuning that by lowering it by 2-3mm each time and going by feel something about having the most control at the downstroke.
I asked my kid to take video of my back and from the side, looks like not much tip toe and pelvis instability leh. Though probably not to the level of bike fitters accuracy, i think shd be close enough.....coz i think for the fitters they also fit depends on the situation and what the riders want, no real hard and fast rule.
Also for me i don't ride 1 shot long rides one, usually per day i never get above 60-70km range.
Then after having dialed in my preferred saddle height, i dialed in fore/aft so that i can maintain a 45 deg bend forwards without having to hold onto the bars....... so for me i did that and tested with 1 min duration without having to hold onto the bars with a 45 deg front bent. I think that is the correct thing to do right?
Yeah i know after changing crank length, need to adjust the height + a bit of the fore/aft again, no issue ba.
No i dont have overworked or cramped hamstrings or for that matter any cramped muscles. No cramps while sleeping also. For reference, i train a fair bit of the hamstrings with bands and my current strength, while still not able to do a nordic curl from the ground up, i can slowly curl downwards (ie negative rep) comfortably past 45 deg before i fall towards the floor.....or i can maintain an isometric hold @ 45 deg no issue. Coz i train all muscle groups for running purpose also ba.....including things like hamstrings, anterior tibialis, TFL.
Not sure if it might be helping a bit in this sense as well to mask....
But one thing is for sure, earlier this year i have changed my foldie to 165mm cranks (from 170), that one is quite easy and cheap, Litepro 165mm is only $40 from Taobao, and i just asked my home biz pal to change for me @ $15 only. So far so good, no issue.
So that is why i have confidence with 160mm, this is for my case.
But ultimately, is hands itchy also.....this is the cheapest and best bang for the buck upgrade that i can think of now.

ie not really intending to spend anymore.
Also a bit strange when u say your back is inflexible but yet can go flat with knees near sternum.
Yeah coz of slightly old age i think, there is a bit of a hunched back around L5 disc, or about middle of the spine...that is whenever i am standing or sitting. My posture (shoulders/scapula) also a bit rounded forwards.....that's also one reason why i gotta do posterior/back muscles training with pull exercises, coz i train a fair bit of push ups so gotta like balance it up.
But just now i just confirmed while lying down flat on the floor, the upper legs (ie femur part) can be bent all the way and touch the chest/ribs. Or put it this way also, i can sit on my chair now and both legs' femur i can bend and touch tightly against my chest but that is with the knees fully bent as well.
However, i cannot do this while in the sit n reach position, ie knees fully straightened. So it's due to the hamstring tightness restricting the ROM.
Like this , no issue my upper leg / thigh can touch the chest.
I have already tried the nerve glide / nerve floss........ and stretches like this. Many months liao. Not working for me.