Current release of RaspiOS (we are on Debian 10 Buster) uses dhcpcd to manage networking (see Raspberry Pi TCP/IP Networking). This means that:

• Use /etc/dhcpcd.conf file to define static IP address or fallback IP address (if DHCP fails). The same file is used to e.g. disable WiFi (wlan0) interface.
• Do not use /etc/network/interfaces for static or dynamic IP assignment.
• WiFi ESSIDs and passwords are in /etc/wpa_supplicant/wpa_supplicant.conf.
• Do not install Network Manager.
• If a gaphical interface is needed, use lxplug-network.
• If some package installs the systemd wpa_supplicant.service, disable it or you will have two conflicting wpa_supplicant processes running (execute the systemctl disable wpa_supplicant.service command and reboot).

I want the eth0 interface to have a static IP address beside the one eventually assigned by DHCP (i.e. I want an IP alias on eth0:0). This is useful when I wan connect to the host even in an unknown network. To achieve this when dhcpcd is running, it is necessary to create an user defined hook script. See this page for more details: How to configure an IP alias with dhcpcd.

We measured the transfer rate from other home NASes, just to figure out how much time is required to transfer 500 Gb of data. The receiving NAS is the Raspberry Pi 4 with a Seagate 4 Tb IronWolf 3.5 inch hard disk connected through the X835 USB3 interface. The network is limited to 100 Mbit transfer rate, due to the limit of the LAN switch.