A couple of options:
NanoPC-T4[0] is based on RK3399, with a big.LITTLE CPU (Dual-Core Cortex-A72, Quad-Core Cortex-A53) with 4GB RAM, supports NVMe and a fanless configuration.
ODROID-HC4[1] is based on Amlogic S905X3, with a Quad-Core A55 and 4GB RAM (no WiFi, only 1G Ethernet) and two SATA ports, which make it well suited for entry-level NAS.