@kiff 

What is the current network diagram? How do you connect these devices together? 

When the stream cuts out, can the TV still access internet? Can PC ping it with its IP address? 

Try to disable fast roaming;

To do a comparison analysis, it is suggested to clear the high priority rules first; 

After that, you can setup application priority as standard to give it a go.

Any updates, please let us know.

Best regards. 

@kiff 

First with the firestick connected to the Deco, you can disable mesh technology to verify whether it works.

If still the same, then please disable fast roaming.

Besides, how many Deco M5 do you have？ From the network diagram, there is one Deco existed, but from your original description, there should be 2 or more. 

Good day.