May I introduce my Assistant to you?
I didn't use the account Cayin Support, her name is Silvia and, among other duties, she helps me out on Facebook and HeadFi forum tremendously.
Silvia has approached the problem based on one assumption: the RK3188 MCU is the bottleneck of the problem.
N5ii can playback music on NAS directly but there are limitations. MP3 and CD quality (16Bit/44.1kHz) WAV and FLAC files are perfectly alright, but when we move up to 24Bit/96kHz, or DSD, the playback sill start to display stuttering or even hang up completely.
A lot of users will assume the stuttering was caused by slow WiFi but from what we understand, the problem probably related to MCU processing power. When N5ii connected to NAS and start to read the folder remotely, there are a tot of task involved already, and then when it start to play a song on NAS, N5ii will start to read from NAS and write into buffer at the same time, so more processing power is involved. When we need to playback a Hi-Res file, the data involved in the buffering process will increase exponentially and eventually, the MCU can't handle the "WiFi connection + networking overhead + remote access + real-time buffering of HI-Res files" all at the same time, so it crashed in the worst case scenario.
When we test N5ii with NAS, our testing library was relatively small when compare to your 800GB, 200+ folder Music Library, maybe the size of the music library has created a processing power drain in the MCU and eventually crashed the DAP. I think this is something that worth investigating. I am not an expert on networking so I shall pass the case to our Engineers, will probably take a while to schedule a test but Silvia will follow up on this investigation.