Your problem, unfortunately, is that your mixer is not capable of doing that. It's not what it was designed for. It was designed as a mixing board for
live sound management. It is only an "audio interface" to the extent that it has a USB output that can send its 2 output buses to a computer enabled with recording software, rather than to two powered speaker arrays, meaning the left and right side of a PA. That's the inherent
concept of a mixer. It takes a bunch of signals and combines them into a smaller number of outputs, usually an even number. The only case in which a mixer makes sense in recording is when you have more signals than you have channels. Drums are the main culprit, as it is the instrument that wants to send out the most simultaneous signals.
The simple answer is- you have the wrong piece of gear to do what you're trying to do. What you need is a multichannel audio computer interface that has however many channels and preamps you need. How many you need is determined by how many open mics you intend to use at the same time. Short of some *really* expensive units, there are very few that will give you more than 8 simultaneous inputs with preamps, although some of them will allow you to combine, or "piggyback" (2) 8 channel units for a total of 16. Be aware that many units have a bunch of inputs, but only some of the channels have preamps, which means that you need an external preamp for every channel you want to use that doesn't have its own preamp.
That's something a mixer *is* good for, because, at least, it has a bunch of preamps with line level outputs, so you can send the line outs of the channels on the mixing board to the channels on your interface that don't have
preamps. Lately, there's the beginning of a trend toward standalone recorders that are also recording interfaces. These *can* do what you are talking about, up to the limit of how many simultaneous inputs and outputs they have. Here's a not-particularly expensive unit that would do your job-
Zoom R16 | 8thstreet.com | Call 1-800-878-8882 | Most Orders Ship Free!
t has 8 preamps, but only 2 of them have phantom power, which is required by most condenser mics. Dynamic mics, which are used often on drums, don't need that, so you would probably use the two phantom-powered channels for 2 condensers as overheads, and the other 6 with dynamic mics for snare, kick, ride, high-hat, crash, and toms.
Here's an 8 channel audio interface that isn't a recorder, but has somewhat better preamps. It will give you phantom power on all 8 inputs:
Focusrite Scarlett 18i20 | Sweetwater.com
Hope that helps. I'm sorry your board was designed to do something other than what you want to do. If you bought it for that purpose, I feel your pain. Best of luck.