A value of 48 (0x30) for configMAX_SYSCALL_INTERRUPT_PRIORITY is reasonable if your MCU uses 3 or 4 bits of interrupt priority. Remember that configMAX_SYSCALL_INTERRUPT_PRIORITY is shifted, just like BASEPRI.
If your MCU uses 3 interrupt priority bits, then 0x30 corresponds to interrupt priority 1 (because 0x30 >> 5 is 1). This would work correctly given the interrupt priorities you listed.
If your MCU uses 4 interrupt priority bits, then 0x30 corresponds to interrupt priority 3 (because 0x30 >> 4 is 3). This would also work correctly given the interrupt priorities you listed.