Playback Error. Playback failed due to a fatal player error.

[TheDreadPirate]

Transcoding only happens when the client doesn't support directly playing the media.  So it won't always occur.

Also, your CPU does support HEVC and HEVC 10-bit so you can leave those boxes checked.  And while your NAS's CPU supports tone mapping, Intel's driver deprecated support for the kernel that is running in Synology NASes, currently 4.4.x IIRC.  So tone mapping needs to be disabled if you decide to just stick with running it in Synology directly.

Running a VM and passing in the GPU into the VM would get around that since the VM would run its own, newer, kernel.