The problem is - your storage device isn’t a server and when transcoding is required you are done - take a look here:
Can hardware transcoding save the day?
Dunno - might save half the day, but you’ll need an Nvidia Graphics card (one that’s approved for your machine and Plex) and you’ll also need a Plex Pass to find out if it helps - and how much.
@FordGuy61 may be able to help you more.
I have storage, but it’s only role is storage. A Server handles any heavy lifting and Transcoding is the heavy lifting in the Plexiverse. There’s nothing heavier.
As it stands right now Direct Play is the only way to survive with your equipment. Obviously your home videos won’t Direct Play and require Transcoding and therein lays the problem.