A for why Orochimaru didn't enhance himself with what he had... He might not have been even remotely ethical about his experiments, but he was a man of science.

You don't experiment on yourself if you can help it.

Also... those hundred or so test subjects that all died and turned to trees when he grafted them with Hashirama Cells.

"perfect the process and make sure it's safe for me" or "you know what, this method is tried and true so I'll stick with that."

...And yes, this means that Senko's kind of a bad scientist.