Ground Strap vs. Ground Cable

All of the above sounds good, but here's the thing..........

just how do you equate cable size to a flat ground strap? Is it equal, better, or smaller?

"Flexing." Just how much should a ground strap have to flex? The starter cable flexes, maybe more, and it's not flat strap

So far as fine vs not so fine wire, you may be thinking of "skin effect" the so called phenomenon where AC (not DC!!) current tends to flow only on the outer parts of the conductor. This gets more pronounced as frequency of the AC increases, and is why, in such things as high power RF transmitters and amplifiers, that large flat conductors, hollow tubing, and silver plating is used