I have 2x 1Gbps WAN connections and trying to load balance them so that I can achieve 2Gbps nominal. My main router is a ubiquiti UDM-Pro and although it will load balance, it will not exceed the max speed of 1 port (Confirmed by months of support requests and testing). If at all possible, I would like to make the device as transparent as possible for things like Port forwarding, firewall rules, etc.
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.