bonienl

Maximum size is the largest memory size your motherboard can support.
Usable size is what is actually available to the system.
 

I have been using Dune media players since 2009, and they work wonderfully.
The older models had only fast ethernet (100Mbps) connection. Dune uses their specific algorithm to read files from SMB, this is used when you enable “fast smb access”. Btw this setting does not exist anymore in nwer versions, which are optimised by default.
 
The graphic shows a constant reading (left) vs a lot of fluctuation (right), the latter part explains your stuttering.
 

Yes, disable bonding and enable bridging for eth0 (1G interface - br0) then add eth1 (2.5G interface) as member of the bridge.
Connect your PC to eth1 and it should get internet access thru the Unraid connection.