@tmcthree 

Hey

You are correct in what you are reading.   OFDMA is a technology brought out in WiFi6 to increase the capacity overall for congested networks, by congested we are talking 60+ devices on one AP.  Being honest, with 18-20 devices it wouldnt be any advantage to you as you are probably not flooding the airway enough to warrant this.  Even then the early feedback from users is a 5-10% increase in capacity at best    There are reports that when OFDMA is enabled the speeds per device drop, this is not un-expected, you cant have capacity and speed together.   This also requires the device to support the creation of the subcarriers.

MU-MIMO will help, it allows mutiple devices to talk at once which does increase overall performance.  Some AC range (WiFi5) devices support MU-MIMO

WPA3 - will only work if the device supports it, likely most will simply operate at WPA2 AES

1024-QAM  -  Again device depending this will offer a great increase in throughput, however its early days and would would likely need clients to update to support it.

In Summary..

WiFi6 is a step in the right direction for capacity and multiple devices, however its not that ground breaking just yet.  We are early days and still on the first release of devices, this will get better buts its a price premium at moment.   

If you are after better WiFi and have a few hundred $ £ to burn its likely going to help a bit, your WiFi5 clients are always going to be the issue for you.   If you are going to upgrade short term cheap (2 years?) it might be better to disable the WiFi on your router and buy a EAP245 /  265 something like that for $£ 80-100.  That will give you MU-MIMO, I have one of these (EAP245) 30 clients connected without a problem, as these are business grade they will support 60 clients with ease! 

Be a cheap upgrade until Wave2 of the WiFi6 devices come out and the performance really increases, but thats gonna be 2022 at earliest! 

@Philbert Thanks so much that's really helpfull.
I'll do some more research into MU-MIMO. I get the impression that its a bit older than OFDMA so more of our devices will support it. (Although I think the majority of my stuff still doesn't).  

@tmcthree 

Yeah MU-MIMO came out in Wave2 of the AC range, so its still current generation enough.

For 20 devices any good AC capable device should be more than enough for your needs.