I am thinking about adding a router that supports OpenWRT like Archer C6 and use my existing isp router as a bridge. I also want to run wireguard on the router.
Anyone have any experience regarding it?
Well, performance can't really increase beyond what the hardware is capable of so it's nearly identical to stock BUT the stability and feature expandability is hugely better.
Also you can find a plethora to tutorials on youtube for your task and there's documentation as well.
Every router which is sold in last 10 years, is capable of saving internet credentials using PPPoE option.
I am guessing your router is configured as Dynamic IP and then login using the ISP webpage.
Switch the connection mode to PPPoE with credentials and you will never required to open ISP webpage.