In order to change blower speeds you should be referring to the manual for heating because if you don’t keep that heat exchanger the correct temp it can crack.
this can also happen from poorly designed systems
I am trying to figure out your plan but it’s tough to see from the pictures
Replacing metal duct with flex will reduce velocity and noise. A 8” hard duct will deliver more air than a 8” flex duct, especially on a long run. Typically for noise I always like 5’ of flex especially on higher end projects; we just up the duct size sometimes