Dragon Ball: Did I Become Dr. Gero from the Start?

No heroine, no saintly protagonist. The male lead is Dr. Gero.

From the start, he transmigrates as an old Dr. Gero. With help from Robert, the very first android, he uses Son Gohan's S-cells and a fusion agent to turn himself into a Saiyan, then recruits Raditz and Son Gohan.

Later, he uses his advanced technology to master the abilities of all kinds of strange aliens, enhancing his own Saiyan transformations and replacing Frieza as the emperor of the universe.