Can I use M.2 NVMe PICe SSD with a heatsink?

No, there is no space left inside of Wokyis M5 to install a heatsink.

However, the SSD cover for the M.2 NVMe PCIe SSD on the rear of Wokyis M5 is a rugged aluminium component with grooved design, which technically functions as a heatsink. You only need to put the silicone thermal padprovided in the packing box between the SSD and the SSD cover to ensure the heat dissipation.