Asus routers have a “Media Bridge Mode” that does exactly what you want

But obviously if the wireless signal is poor where you’re planning to put the Proxmox server, you shouldn’t expect miracles.

CGNAT affects accessing your LAN from outside your network, i.e. over the Internet. Traffic within your own LAN is not affected whatsoever.

There is no such thing as cat 6E cables. Also, matching the wifi number to the cable category number is unnecessary and pointless.

Cat 6 will support 1 Gbps, 2.5 Gbps and 5 Gbps to a distance of 100 metres, and 10 Gbps to a distance of ~55 metres.

Keep your cat 6 cables and don’t waste your money.

Set the secondary router to access point mode. Then you’ll just have one big network.

You’d put the Xfinity modem router in bridge mode and connect your own router

Yes, port forwarding will work on the router. The Xfinity box will be acting as a modem only, so will forward all traffic to the router and it’s up to the router to do everything else.

Go to add printer, then click my printer wasn’t listed, then add by name using the share path from your screenshot

OPNsense and do PCI passthrough of the NICs

Might just be India or your specific ISP. I’ve lived in a handful of countries and none of my ISPs charged extra for allowing incoming connections.

Unmanaged switches will not have any settings to change, so you’d need to find a managed switch with this option.

Or keep it low tech: unmanaged switch and black electrical tape.