I have no doubt that John Millius and Oliver Stone had some familiarity with Howards stories and characters. They likely knew that Thoth Amon was Conan's main villain , but chose Thulsa doom because they liked his name better. It could be as simple as that. And I agree James Earl Jones could have easily played Thoth Amon.
Another actor that would have also been good choice to play Thoth Amon is Amrish Lal Puri . He played Mola Ram in
Indiana Jones and the Temple of Doom. He's was a great actor and he would have been perfect for the role of Thoth Amon !