The Arris S33 isn’t recommended for Comcast as it doesn’t support Comcast’s mid-split. Buy the Hitron CODA56 instead as it supports Comcast’s mid-split and is cheaper.

A $30 router will have poor NAS performance. Also, I doubt it can act as a NAS when not in router mode.

It is impossible to load-balance a single connection when using NAT, as a connection is defined by the IP address. You will get an aggregate 2GBps if there are multiple connections at once. If you want to overcome this limitation you must own your own IP address and purchase transit from two ISPs, but that is expensive and requires networking knowledge.