understandable when buddha is the only one who achieved enlightenment, thus raising the question as to how anyone else can after untold millennia
That's not getting into every branch of Buddhism having its own spin on things and some just outright being batshit crazy. If Buddhism were ever a coherent faith, it wouldn't have splintered as much. Even if you look at the sects of Christianity, you can see the main difference being the structure of the church and debate about the nature of Christ. You have the oldest branch of Buddhism not acknowledging any other Buddha, Vajrayana's crazy sex shit, Korean Buddhism itself stemming from the Koreans buying into it but acknowledging how fucking messy and in need of correction so much of it was, and the Jap's sidelining all the suffering bits or viewing it from another perspective entirely.