A banana pi is not going to have the processing power to properly route that much data.
You’re looking at an Intel nuc at minimum.
If your internet is 5gbps and the router only has 2.5 gbps ports, you’re going to be capped at 2.5gbps unless you start getting really over complicated because the router expects ONE incoming port. There ARE ways to support pairing two Ethernet ports together, but your ISP’s modem/router won’t support it because “fuck you for having your own hardware”
Also because they’re cheap.
If your switch is 1gbps, you’ll be limited to 1gbps.