The WiiM Pro Plus hardware is purely the player, it needs controlling software to instruct it what to play and from where. In the WiiM world the WiiM Home App is that controller but I don't believe it's available on the Amazon app store (unless you've installed the Google Play Store) so this will need to be installed on another device. There is other controller software out there, but the WiiM Home App will still need to be installed somewhere for the initial setup of the hardware.
If Plex can run on the Fire HD 8 and serve music over UPnP then yes, the WiiM Home App on another device will be able to see the files stored on your tablet and instruct the WiiM Pro Plus to play them from the Fire tablet.
You can test this setup now by installing BubbleUPnP onto your controlling device (or mconnect if on iOS) to see if it can discover and play from Plex on your tablet.
I've never installed Plex on a tablet, but assuming it's not just server software it may also be possible to play from that to the WiiM Pro Plus over Chromecast, although this wouldn't be gapless.