Holy shit, eh!? 20 discs? Aren't they like $10 each? Maybe you might get 5 for $10, but most every dremel bit seemed to be $10-$15 at home depot.
I normally buy the augers where you replace just the disc, just had a look and my pack has 36, I can't remember what I paid but it was more like $5-10 for that many. The auger was part of a larger pack, but think they're something like $10 in a pack with 3 or so discs. These are called "Cut-Off Wheel No. 409". There's also EZ-Lock products that are excellent for steel (got mine free as part of an online promotion) and more convenient to change but I wouldn't use them on plastic because they're a bit dear as well. See how you go but those solid grinding pieces don't seem to be so good for plastic, they just clog up too fast so better to use the cheaper replaceable discs.
ETA - Just had a look and those have a list price of $6.76 at Amazon (they sell for $5.29) and while they don't recommend them for "sanding" they do seem to work well that way for plastic, and it doesn't feel so bad to have just ruined something that's only 20 cents odd.
I started out with the cutting discs, but found they (the free ones provided in the dremel 4000) were a bit too think to make a proper incision without grinding away the metal connectors. Pretty sure I botched one of the cables so far using the discs
I ended up doing the rest of the cables with a small steel star-shaped grinder. It's only about 2mm wide and is great to make the necessary grooves. Will show with pictures soon.
Sidenote: Why would manufacturers limit themselves to creating x1 only cables in the first place!? Surely it would be more profitable to use less plastic and allow all sizes of PCIe boards to fit, no?