I think OP would be talking about a third party router. Probably custom built.
But then his first assumption doesn't stand. I only need one router, and it's already provided by the ISP...
I think maybe this is the break down here.
Just because your ISP rents/leases/gives you a router doesn't mean it's a good one. The cost of that router is still included in your services. OP just wants to slap a couple chips on it to make it mine. I don't think it's a bad idea it's just not that viable.