Regarding the router firmware page: Router Firmware - Privacy Guides
What exactly is the problem with using the standard router given by your ISP? My router settings don’t contain anything relating to privacy. And obviously my ISP knows who I am and which IP addresses I connect to, no matter what router or firmware I use. So what’s really the benefit of using an alternative firmware?
As above pointed out many routers collect data and share this data to third parties often labelled as anti virus companies.
Besides that often providers have a backdoor account for better and for worse to the router you get from them giving them direct access to your home network.
Don’t forget the insecurities. It’s not uncommon for cheap routers to run outdated linux 3.x or 4.x kernels with no updates. Hell, the official way to install openWRT on a xiaomi router is with a system exploit