Bandwidth steering helps dual-band clients, like laptops, mobile phones, tablets, etc. to use to less-congested and higher capacity, 5 GHz band. This will result in better WiFi performance for those devices. You can enable and configure bandwidth steering per device in the Unifi Controller.

UniFi Network - Understanding Device LED Status Indicators UniFi Network - Understanding Device LED Status Indicators

The system was set up and is controlled by a local ISP, who we would like to get away from. We have recently had 900+ mbps FTTP installed, onto which we would like to transfer all of our existing network. Physically that is not a problem, and it is not a problem to physically reach all switches, APs and routerboards. If you use an Unifi access point, then you can easily configure one SSID of all the access points. If you want to use another brand, then just enter the same SSID and WPA key on both devices. Only use a different channel of each access point.

Does the access point project my network if I don’t do anything other than plug it in to my lan (through the POE)? In other words, can I use it to function essentially as an extender without any configuration?

802.11ac Dual-Radio Long Range Access Point - Ubiquiti

If mounting an AP inside a normal wall plate is what you are looking for, the In-Wall and In-Wall HD are both good options. If performance is important to you, get the In-Wall HD. It has better antennas and more capable radios. Another difference between them is the number of Ethernet ports they can provide to downstream devices.

You can indeed connect your AC-Pro to the EdgeRouter ER-4, it will need a network cable from the route to the access point. To be clear, the ER-4 can’t power the access point. But with the ER-4 comes a Power over Ethernet (PoE) adapter. You can place the adapter near the router. It has two gigabit Ethernet ports. The 2nd can be used to bridge to another device, or combined into a 802.3ad-based link aggregation.