The Twonky DLNA Media Server (which is a Digital Media Server, or DMS) provides a DLNA media library; you point Twonky at your media directories on the MyCloud, and it goes off and creates a library of music, pictures and video, using various bits of metadata in your media files.
Your TV will need a DLNA Digital Media client, which combines the functions of Digital Media Controller and Digital Media Renderer. I’m sure there will be a DLNA client on the Samsung smart TV. It just may not be very good, either in basic function, or in the user interface experience…
Your WDTVLiveHub was a media server/player. The MyCloud is not; it is merely a NAS, and has no media-playing facilities built in. So, you will need a media player client, as above.
Your Amazon Fire Stick should be able to run Kodi, and access the MyCloud via SMB (rather than DLNA, which Kodi will also support, along with a number of other network protocols). Kodi will then index your media directories via SMB, and create a media library. I use this approach for videos, using an Android media box running Kodi, accessing via SMB.
SMB is a better access method for Kodi, since it allows Kodi to create its own media library, rather than the one provided by Twonky. It can therefore use various scrapers to identify films, TV programmes and music, and provide nice artwork to give a prettier user interface; Twonky has to rely on metadata and artwork embedded in the files, or in the media folders (e.g. CD artwork).
Plex requires a Plex server to run on your media storage device (MyCloud in this case), or on some other computing device that has access to the MyCloud media store, and a Plex client running on some renderer device. In this respect, it works similarly to DLNA.
If I were you, I’d go FireStick/Kodi, as I really don’t think there’s much difference between Kodi and Plex. Or buy one of the thousands of cheap, generic Android media boxes that are available for $30 or so.
n.b. you may see negative press associating Kodi with illegal streaming; that’s nothing to do with the basic Kodi function, but is all to do with add-ons that Kodi supports. You have no obligation to use these add-ons, and they are not provided as part of the basic Kodi installation.