which is the best channel to use for wifi 7 to take advantage of the 320mhz channel width

The "up to****mbps" is what changes the channel width. 

And any should be fine. 6ghz doesn't broadcast as far or through as much material so there's less chance of interference. 

Here are the notes I made regarding channel width (for the Netgear RS700, this should apply for the RS300):

20 MHz = up to 0.7 Gbps on 2.4 GHz
40 MHz = up to 1.4 Gbps on 2.4, 5, and 6 GHz
80 MHz = up to 2.9 Gbps on 5 and 6 GHz
160 MHz = up to 5.8 Gbps on 5 and 6 GHz
320 MHz = up to 11.5 Gbps on 6 GHz

I have my 6GHz wifi on 320 MHz (up to 11.5 Gbps) with no issue. Remember, if you want your 5 GHz wifi on 160 MHz (up to 5.8 Gbps), channel 149 is recommended. If you choose channel 36 for 5 GHz with 160 MHz, your channels will be from 36-64. Because channels 52-64 are DFS channels, there's a chance that your router will be kicked to another channel.

So for my new rs500, I cannot get 320.  Best I can do for 6g is to set it to 'up to 2.9gbps' and it will get to 160.  No other combo of channels/speeds works to get 320.  My ASUS router has no problem doing 320 for 6ghz.  How can I get this rs500 to work at 320?