You need a UniFi Controller for the Management of the Ubiquiti devices. Especially for the UniFi WiFi devices. But you have several options for doing this. From the large solution with a Dream Machine Pro to the smaller Cloud Gatways or the installation of the software on a device in your network (from Windows computers to Rasperry Pi, almost anything goes). You can find the selection of devices on the Ubiquiti homepage under Cloud Gateway. Depending on your needs, you can also replace your router with a gateway. Digitec then has all these gateways etc. on sale.
The Ubiquiti gateways are then very easy to set up. The GUI is very intuitive and for most functions you have a "normal" mode without advanced functionality. So submitting a VLAN, then setting up an internal and a guest SSID for a WiFi is very quick.

No, the flex switch can function "out of the box" like a conventional, unmanaged switch - i.e.: LAN cable in, connect devices, PoE supply if necessary - and you have a functioning network. BUT: Functions such as VLAN, bandwidth/QoS settings, port isolation, monitoring, etc. cannot be used without a controller. In addition, you will not receive any firmware updates. It would therefore be overkill to buy this device for your purposes. You can buy an ordinary switch for your purposes, which is a lot cheaper. Otherwise you're basically buying a Ferrari for a village street ;)