How much difference does a turntable really make compared to the cartridge?
A lot.
Will I hear a significant difference if I upgraded my turntable and kept the same cartridge?
Yes.
Isn’t the cartridge 90%+ of the sound from a vinyl setup?
Yes. It is. The turntable is only 90%. Oh, and the phono stage. A good phono stage is 90%. At least. Tone arm, 90%.
By my count that's 360%- which coincidentally is exactly how much better analog is than digital.