You may not be able to get the colors of certain shades but braided fishing/spear fishing line has also been used by owners. I happen to have a white roll and used it it operates very smoothly compared to the old blinds cord.
I used a vacuum to suck the cord to far end of channels . Then kept next cord taunt to draw the next one through. You don't want them to tangle .
The hardest part is getting proper tension. Particularly if older really tight fitting fully boxed valances that used just a screw and finish washer instead of spool type clamp. That's what I have. The spools may interfere with the fit at bottom.
95 Monaco Crown Royale
M11 400hp, 4060 trans.
Aquahot, Generac Guardian7.5k