WiFi perspective, there is a huge difference in Data Rate vs Throughput. Also, it all depends on client capability (the below page may help you to understand different clients' capability) https://clients.mikealbano.com/ If you want to test the best case scenario, there should be only 1 client (iPhone in your example) on your AP & it should get higher RSSI/SNR to achieve the MCS11 data rate (1.2Gbps - assuming 80MHz, 2 spatial streams with OFDM). Even such data rate, throughput value is normally 40-60% of data rate, as there is a lot of overhead in a wireless cell. Therefore getting 506Mbps (upload direction) indicate you are getting close to the best possible values.
Download speeds are not exactly what you would like to be, I would do further testing by checking RSSI/SNR "show client detail <mac_addr>" CLI output tell what AP thinks client RSSI. If you can get those details from Apple client itself, that is what matters and that determines the MCS rates the client go with.