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.

Mikrotik Wireless Wire

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.

So the maximum speed tier you can subscribe to will be 100/40

Honestly, a great modem router for this is the Telstra Smart Modem Gen 2 (model number DJA2030 or DJA0231). You can often find them unused on Gumtree or Marketplace for under $50. They are not locked to Telstra (work fine on Aussie Broadband) and come with a built in SIM card that will kick in if NBN is down.

That would be my recommendation.

1000/50 is not available on VDSL NBN connections (FTTN, FTTB, FTTC)