For purposes of transmitting small amounts of DC power as you mention, just about anything down to a rusty coat hanger can usually be made to work, so long as you’re not trying to transfer data over the same cable. The smaller the conductor diameter the higher the losses encountered over long runs, and at some point for a given piece of equipment, those losses become unworkable. Where exactly that point is for your equipment, I can’t say.
Free-hanging barrel connectors on hand with a 2.1mm ID are here, and the two Adafruit products you mentioned would seem adequate for the other case. As for your thoughts on choices of cable material, those wouldn’t necessarily be my first choices as they’re relatively expensive/overkill for the task. But if you’re working with materials at hand, go ahead and give them a try…