I've been using a similar solution, but I had quite some problems using AP and client mode at the same time. The WiFi chip inside the RPi Zero W (BCM43430) only supports AP and client mode on the same channel. If you setup an AP first, then connect to a client network, hostapd will update the channel of the AP, but clients will notice a disconnect. There are also issues with the firmware blob (see https://github.com/raspberrypi/firmware/issues/1403) in AP mode that haven't been solved yet.