Have you considered the Fractal Node 304? Uses standard ATX PSU and accommodates up to 6 drives.

Depending on how small an ITX case you are looking for. Here are 2 more pretty compact ones for NAS use.

Edit: disregard below, saw now the board is M-ATX

Lian-li with internal hotswap https://www.amazon.com/gp/aw/d/B005FYK7KE/ref=psd_mlt_bc_B005FYK7KE

Fractal design node https://www.amazon.com/gp/aw/d/B009LHF4FO/ref=psd_mlt_nbc_B005FYK7KE_