I want to buy a Raspberry Pi 4 Model B to use it as a home router and WiFi access point by flashing OpenWrt. I'd buy also a Gigabit Ethernet switch to create a local LAN for wired devices.
I found this guide but it uses an external Ethernet-to-USB cable and so creates an additional eth1 connection on the Pi that I think could be avoided if I connect both the Pi and the ISP Router (modem) directly to the Switch, is that correct?
Then, how can I configure the Pi to also manage WiFi (act as Access Point too) in addition to that guide?
Otherwise, could you provide a detailed guide to set everything up for my case?
Then your Pi should be configured only as an access point if you want to do
Your best bet would be to buy TP Link UE400 USB to Ethernet adapter and connect the gigabit switch to the RPi through the UE400. Configure the Pi as a router and put the ISP router to bridge mode.
You can do SQM at Gigabit speeds using the Pi as a router. Do keep in mind that the wifi in the pi is very weak. You can buy a cheap wifi ap and connect it to the pi, this can let you get better wireless coverage and speeds.