I have two internet connections and I want certain devices/users to always to go through the specific connection while the others would be able to use either WAN in a typical load balancing setup. The point is that one WAN has a monthly cap and the devices forced to use that WAN should not have any internet once that monthly cap is reached.
I believe a Dual WAN router is the only device capable of getting me close to this functionality. So, I'm looking at this TP-Link.
I thought I could achieve this through static routing, but I don't see any way to apply the policy based on the source MAC or IP Addr. Is this not possible even with a Dual WAN router?
The feature you are looking for is "policy based routing" (PBR), so you have to find a router that support it.
Product recommendation is off-topic here, but among the various options, you can use a free network OS like VyOs
It runs on any x64 platform.