welcome to Plex and the forums.
first, you should read @ Plex, 4k, transcoding, and you, the whole thread, and you may need to re-read multiple times to fully understand.
you may also want to check out other threads on that particular nas, my understanding is that it can do hardware transcoding, but it is not exactly a powerful beast. that said, hw transcoding requires plex pass (you can subscribe for 1 month to try it out).
https://forums.plex.tv/search?q=Synology%20218+%20transcoding
ideally, you should serve 4k to only 4k clients, otherwise, you are just making it difficult for yourself.
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relay is limited bandwidth. you should avoid wifi for the server, and you should ensure that all your local clients are all on the same ip network. Plex treats local and remote streams differently (which can be configured in both server and client settings). You can use the plex dashboard or 3rd party tools like tautulli to monitor the active streams.
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no idea, not enough information provided. personally I have had nothing but bad like with fire devices, I avoid them like the plague. but others seem to have no problems and like them, so your milage may vary.
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the nas has a low powered cpu, and synology has its own internal transcoding engine (as far as I understand) that is separate from what plex uses (and plex requires plex pass for the gpu/hw transcoding, which you do not appear to have). So no, the cpu is not strong enough for 4k transcoding on cpu only.
DIRECT PLAY (ie no transcoding) is the only way you will be able to play 4k (see the 4k faq linked above) without any load.
yes, you are sort of barking up the wrong tree. there is zero benefit to using 4k on non-4k devices. and trying to transcode 4k can lead to a whole new set of problems.
4k is neither simple, or ‘plug and play’.
some other links
