Better presentation of musical overtones, amplitude, pitch, timbre, texture with a wider/deeper soundstage (if this is possible) than I have with my existing DAC. Under $10K, used is OK.
If you’re not opposed to a tube DAC I’d give Lampizator a hard look given what you’re looking for. For solid state the Denafrips Terminator ll with either the Gaia or Hermes DDC (gets you i2S, clock, etc.) is another strong option. Just a couple to consider and either should be a considerable step up from the Gustard on every level. Best of luck.