There doesn’t seem to be a way to shut off the 3v3 voltage regulator as it comes from a separate IC U6 and not the DCDC of the chip, according to the schematic. Charging characteristics of XIAO_nRF52840 and XIAO_ESP32C3 - Products & Technology / XIAO - Seeed Forum (seeedstudio.com)
If some of your peripherals draw less than 15mA, I can suggest connecting the ground and other pins of that peripheral to digital pins, and then setting the digital pins to HIGH right before deep sleep. Elsewhere it is suggested that “The max GPIO sink/source current is 15 mA. This is max both from one pin or as the total for all the pins (entire package).”
I was able to remove most of the quiescent current from a connected Neopixel by doing this, going from above 200uA to around 40uA in deep sleep.