Comments of chapter undefined of Siphon -The Flash

August heart ended up perfecting the velocity serum and making it permanent for himself and using tachyons to boost himself further, which turned the normal blue lightning of the velocity serum into white lightning. So it's entirely possible that if Nathan absorbs enough speedforce infused dark matter, he'll reach a similar state where he won't have a speedforce connection, but he'll still be as fast as a regular speedster. Although if Nathan has a limit of how fast he can get with speedforce infused dark matter, or the team decide to stop him from absorbing from Barry for whatever reason. You could always just have a plot where Nathan realises he needs to be faster and ends up creating his own velocity serum. If Hunter Zolomon could do it, then Nathan probably has an even much better chance of creating his own speed drug.