Purchase advice: Synology 10G model. Hardware acc. transcoding necessary for AppleTV 4K?

plex has been developing a new player which is compatible with more types of codecs and features.

as far as I know, it should be enabled by default, but if not, there would be an option within the apple tv plex player to enable it.

the alternative is the legacy player, which is not as compatible with as many codecs/files, and is being replaced by the new player as indicated above.

(edit: I failed to observe which user was which, this comment below was (mistakenly) meant in response to ekohn, all else should still apply to cake376 though)
it appears you already have plex pass, so if you have not already tried plex, you should be doing so with the equipment you already have, before making any further purchase decisions based on plex.

if the content you have can already direct play on the equipment you have, then you may not need to upgrade right away.

plex start @ https://support.plex.tv/articles/categories/intro-to-plex/quick-start/

regarding 4k content, you may want to review the thread @ Plex, 4k, transcoding, and you for a lot of info and background.