iPerf3 actually sends data TO the server and does not download it from the server unless you specify the -R flag on the client.
If you're not testing from the wireless client with the -R flag, then you're actually testing upload speed, not download speed. And if you have many clients, you're contending for the access medium. So first, test from the wireless client with the -R flag set. If it is an Android phone, then specify the -Z flag too, as the zero-copy mode of operation frees up a lot of cpu-cycles that otherwise decrease your real throughput.
Have you got any QOS running on the router? Switch it off if you do.
Also, a cursory google search reveals that for most people the Moto G6 tops out somewhere in the 100 - 150mbps range. So you're limited by your wireless client, not the router