About as thick as a good clear gravy you'd pour over your turkey at thanksgiving. You can physically see that consistency in my gel mix photos. And if you have a look at the description under the photo of me at the table with the ingredients, you can see how much corn starch I applied. The corn starch is what thickens the mix into gel form. If you don't want yours as thick, just use less corn starch. And keep in mind that I used "heat" in my mix, which is required to get the corn starch to effectively thicken the mix.
As to your comments about dunking, it would be best I refrain from offering advice on that. I simply don't have experience with dunking entire cases myself. But my experience with both liquid and gel forms of RetroBright suggests that you may have a hard time to perfectly "seal" the inside of your Mac's housing.
Best of luck to you. Hopefully more among us here at 68kMLA will make the time to deyellow. It's worth it in the end.