Sitting up and further behind the bottom bracket does seem to work the hamstrings more than the thighs. I tend to ride that way as I’m long of back and short of leg and that keeps me better balances in the bike.
When sat on the bike with hands on the hoods, can you let go of the hoods without falling forward?
With hands in hoods, what angle are your arms?
When you pedal do your hips rock or is your bum firmly planted on the saddle?
Do you find yourself using your legs to keep you in position on the saddle?
When you pedal, are your feet flattish at the bottow of the stroke or do you pedal with your toes pointing down?
I'm not sure on my arms, I'll get someone to look the next time I'm on the bike. I don't think I rock, or hold my position with my legs. Toes slightly down, yes.