500+ Mbps is really good for WiFi, really anything over 300 Mbps is more than enough for phones/laptops doing normal tasks.
For comparison, a 4k Netflix stream uses 15Mbps, and a 10GB file would download in just over 4 minutes on 300 Mbps
For devices that need low latency and more bandwidth, wired is the way to go
AT&T fiber ONTs don’t have a bridge mode, so adding a separate router would give double NAT
That being said, just use the ONT/router as the default gateway, and could use it for dns/dhcp or run your own instead. Can run all your own infrastructure and just disable services on the AT&T router that you want to run yourself. Disable WiFi and use your own access points, etc.
For accessing your services remotely, use a vpn like tailscale or zerotier, or set up cloudflare tunnels for publicly accessible services.
Fiber is better than copper, and the extra upload is absolutely worth it.
This, just pay for it if you want to use it outside your home. It’s absolutely worth it, ports are outbound so nothing to open up on your firewall, and can use yourself or share with other’s Plex accounts as needed