It doesn’t hurt to leave it on, if the client doesn’t need it, Plex won’t kick it in. But this way if it does need it, it’s there ready to go. Particularly for audio transcoding which software transcoding should handle just fine. But if you aren’t transcoding then “higher end formats” of your files doesn’t really matter - it’s just reading files not processing them.
It might help to use PlexDash on mobile or just check your Dashboard to see how the files are playing back while they playback via Plex on your client to get more details about what’s happening but your tests does seem to indicate you don’t really have to worry about transcoding capabilities.
Wireless vs Wired for playback… usually you’ll be fine with wireless in most cases depending on devices; many don’t have any better than 100M for wired jacks but can have 200-400Mbps on modern wireless. Full size Bluray rips are usually 40-60Mbps so typically 100Mbps is fine though some bitrates can spike up (high volume scenes with like confetti or snow with panning or tracking for example), but there’s a bit of buffer in there.
Wired can be snappier because there’s usually less transmit management so that might be a consideration there. Wired is always more stable and consistent but sometimes the inconsistencies of wireless aren’t really noticeable in most real world situations.
The lack of lag you see with “force direct play” is probably from skipping any client checks. That’d be my best guess anyways. Was lag very significant or just a second or two before starting? I typically experience a brief spin-up to 100% on every file myself and it’s usually related more to the client than the server\network - my weak Roku takes a moment to start playback, my computer takes none.
Direct Play and Direct Stream are kinda two facets of the same thing for playing back files in supported client formats without having to transcode. Direct Stream might work - without transcoding - where Direct Play might not; Direct Play says “here’s the file, have at it” while Direct Stream says “here’s the video stream and here’s the audio stream”. This is useful if a client supports AAC audio but only in MP4 container and not MKV; Direct Play would flake because the client would say “no can do” but Direct Stream would work because it would feed the two compatible streams and the client would say “I can do that video and I can do that audio so that’s fine” so you get full quality with no transcoding, just might not get chapters or such. If I’m recalling those functions correctly anyways. Force Direct Play is “native” playback and should be the same as playing from a USB drive or DLNA.
Sometimes on my Roku if I have a file acting up with Direct Play - like where pausing\rewinding throws it off - but Direct Stream will work just fine which tells me its a codec\container\client mismatch; I just have to flip that setting. Can’t tell any difference in quality because it’s the same quality.
If you know all your digital rips are fully compatible - yeah, turning of transcoding and forcing direct play will probably lighten the hardware load. Since you don’t have hardware transcoding on that NAS, and software transcoding can do audio transcoding just fine, you might find a balance there with those settings and avoid needing to buy new or additional hardware. If your files direct play you only need Plex for its metadata and organizing and GUI options really - which is totally fine! If you want things even lighter you could probably get away with straight DLNA - no transcoding, just browse and play the files from the server - but the client side of that is very boring and stripped down compared to Plex\Emby\Jellyfin but it’d also likely work.
Hope that helps, and if I’m off on any of my understandings on some of these aspects someone will know better and we’ll both learn. 
