Synology as PMS with Chromecast or Xiaomi Mi Box

Hello!
I've read a lot of threads here but still seems not getting some key points.
I have Sony Bravia & Synology with PMS. I can't playback any videos as I always get an error about transcoding. I've tried to make PMS settings to play movies without any transcoding so that Sony will handle it with its Plex Player (Opera app). But no success while same move didn't have any problems via DLNA.
Currently I'm thinking about buying Chromecast or Mi Box with Android TV and Plex app.
But I can't get if any of two will be able to transcode movies from Synology so that I won't have any compabilities issues with file formats.
I also have 5 years WD tv live that can play 99% of file formats but I don't like its interface. And hope that new devices can do more (100% of file formats).
Please advise!