Select polyfuse and diode to match each other
The polyfuse should reliably break the connection at currents that the diode can still support. Might need tests at different current levels.
We probably can underrate the polyfuse a lot. It only needs to be able to kickstart the system. On the other hand: AFAIK polyfuses are not meant to trigger a lot.
The assumption is that the PMIC will carry all the current up to 500 mA itself, as long as the difference of VUSB and VSYS is < 0.7 V