This whole thing keeps coming around to "maybe he did a Kaioken".
But does "The Kaioken Theory" explain everything that comes
after? No, it doesn't. And that's the main point: "
why are the Super Saiyan forms nowhere near 50x the base levels later on?".
All the things we see, from the power increase from Fat Buu to Super Buu, to Babidi's reading of Goku's energy, to Trunks' and Goten's fight with Android 18, and Vegeta's interaction among that, ALL suggest that Super Saiyan is only around 3x the base level.
And Kaioken can't explain that; Trunks, Goten, and the rest can't even
do Kaioken.