You can use your own VPN service on the router by configuring the integrated VPN client in the Deco app. The router supports protocols such as OpenVPN, PPTP, L2TP/IPSec and WireGuard, so you can import configuration files or keys from your VPN provider and protect all devices on the network via the router without installing apps on individual devices.
Yes, the channel can be changed. I don't know what it looks like after a restart.
But: The app notes the following: "The selected channel can be changed during network optimisation"
Well, as long as you are in the range of the WiFi. Being between 2 buildings, likely you are only going to use the 2.4Ghz band, because the 5 and 6 Ghz do suffer of obstacles. Unfortunately, the 2.5Ghz is also the band on which you will have the lowest speed 688 Mbps vs. 2882 Mbps of the 5 Ghz and 5765 Mbps of 6 GHz.
Another thing to check is not not have double or triple NAT, as you likely are going to have one router from the internet provider-Deco BE65 (1)-Deco BE65(2)-router-final device. It will work as long as you are in the range, but this is not an ideal setup.
Yes, you can connect an Ethernet switch to the second mesh router to provide additional LAN connections. This is particularly useful if you want to connect more wired devices. Make sure that the switch is compatible with the Ethernet backhaul feature of the mesh system to ensure a stable connection.