This is a very thought provoking thread. Just wanted to comment on a couple of points raised:-
1. Crank length. Here's a really good article summarising recent thinking on the subject of ideal crank length vs leg length:-
https://bikedynamics.co.uk/FitGuidecranks.htm
Basically most people would probably benefit from slightly shorter cranks, especially if focusing on endurance cycling. Being quite tall and my current bikes both having 175 mm cranks I very recently discussed this with a very experienced fitter in regard to a new bike build. He was very much in favour of shorter cranks (for the same reasons summarised in my link above) and advised me to go with 170 mm cranks
maximum. For reference my inseam is 88 cm and I'm 184 cm tall.
2. Flexibility and stretching. This idea that you can somehow increase your range of motion from stretching doesn't seem to stack up with my personal experience and the science is not exactly conclusive either. Maybe stretching helps to maintain what natural flexibility you have, but I couldn't touch my toes without bending my knees when I was 10 years old and now aged 53 it is exactly the same. I can reach roughly half way down my shins as I always could. I do stretch and I maintain my full range of motion. It just isn't that much based on touching my toes. Again I discussed this with my fitter and he said it's quite common. I just have relatively short hamstrings, simple as that. My back is fine, I don't have any joint issues. It's just that my natural proportions don't allow toe touching!
3. Saddle set-back. I find that set-back makes it easier to reach further forward with a flat back. Try standing with your backside up against a wall and see how far you can bend forward from your waist. Now take a step forward and try again. Once your backside is free to move rearward you can bend forward much further. So moving your saddle back relative to your feet might actually make reach easier. Slightly counter-intuitive but the wall exercise demonstrates the effect quite well.