Jon! Glad to see you are making progress. I've only had my Model 24 for a week now so I'm not much further along than you are. I have a Windows 10 computer as well as an older (2007, OSX El Capitan 10.11.6, GarageBand '11/6.0.5) iMac and on both I user Reaper.
I'm also still learning the routing. Closing the loop on your earlier GarageBand question, I was able to record from the Model 24 by installing the driver, selecting enable multitrack recording and choosing New Track / Real Instrument and then selecting my desired track from the Import Source list.
Regarding mixing out of the box, I too had to use a more powerful DAW, in my case Reaper.
Now, I'm still sorting this stuff out, so I know I will find a better way but for now I have a couple of workflows I've sussed out.
If my goal is to mixdown my DAW session to the Model 24, whether the session originated in the box (combo of virtual instruments and real instruments) or whether the session originated as an import from the SD card of a Model 24 session that I wanted to treat with plugins etc, then I am doing the following:
On the Master Channel in the DAW, I am removing the default track 1/2 output and adding a hardware output of 21/22 since that particular channel strip has the fewest frills and is the catch all for phones etc anyways. On the Model 24, I set the strip to "PC" on the input select.
For each track that I want to send to the Model 24 for mixdown, at this time I'm just adding a hardware send and assigning my desired channel strip on the Model 24. For each track that I choose, I set the corresponding input source on the Model 24 to "PC".
In order to bounce the final mix back into the DAW, I create a new empty stereo track and set the input source to tracks 23/24 (these are the two Master channels on the Model 24). I will arm this track to record and monitor source. I then select a section of the song that is loudest and loop it just to get my levels.
Over on the Model 24, I will adjust the channel strips EQs to my desired settings, adjust the faders etc. When it comes time to record the mix, I will either mute track 21/22 on the Model 24 or uncheck the send to Master checkbox on the output sends of the individual DAW tracks just so we don't record the monitoring channels we set up for the DAW.
Once you are happy with your channel strip settings and your mix on the Model 24, then record from the DAW. The track you recorded of tracks 23/24 will be your mixdown.
If you want to record all of the tracks you sent from the PC/Mac to the SD card, just create a new song, arm each track on the Model 24 you wish to record and then hit record on the Model 24 and play on the DAW. To listen back to what you transferred to the Model 24's recorder, switch the channels you wish to audition to MTR for the input source.
If you want to grab what you mixed in the box, just arm track 21/22 in the above workflow and you'll have a copy of your original in the box mix.
To run individual stereo tracks back to the Model 24 in order to work hybrid, what I have worked out so far is as follows:
Set up the track you wish to work on to use a hardware output corresponding to a channel strip. Set up a new track and set the input source to tracks 23/24. Make the EQ settings you wish to make. Then record the new track via the DAW. The resulting file will be your treated track. If you use this workflow to tracks 1/2 you can use outboard gear. You'll need two insert cables to route the outboard gear. (1/4 TRS male to dual RCA or 1/4 TRS male to dual 1/4 TS male mono) Tip is send, ring is receive.
Also, I've been experimenting with routing to allow sending for channels out of the box to the Model 24 at the same time and recording them. With 4 patch cables you can mix through the Model 24 and back into your DAW.
To do this, I'm sending my 4 tracks out on tracks 1, 2, 3, 4 and back in on 5,6,7,8. On track 1, I select sub and pan all the way to the left. I then attach a patch cable from Sub L to track 5. On track 2, I select sub and pan all the way right. I then attach a patch cable from Sub R to track 6. On track 3, I set the Monitor 1 Aux pot to 0db (about 11 o'clock on the dial - it's marked). I then patch from Monitor 1 to track 7. On track 4, I set the Monitor 2 Aux pot to 0db (about 11 o'clock on the dial - it's marked). I then patch from Monitor 2 to track 8.
For input settings, on tracks 1-4 select PC and for tracks 5-8 select line.
Next, inside my DAW, I set up hardware sends for the 4 tracks I wish to work with to tracks 1 through 4. I then create 4 blank tracks and select inputs 5 through 8.
Use the channel strips on track 1 and track 5 to make your desired settings. Note that because you're using the track 5 line level input, you are routing through the low cut and the compressor coming back in to your DAW. Repeat this process for each of the 4 routes and audition as needed until you are happy with the sound. When you are happy, arm your 4 incoming tracks in the DAW and record.
You can also use the same concept to bus down tracks to tracks 13-22. In a plain old old-school PortaStudio way, you can also use this concept to use the 5 stereo pairs of tracks as buses. Note that if you doing this standalone to the MTR, you will get the original low-cut and compression you printed when you recorded the individual tracks and the EQ and faders will print from the tracks you are mixing down but only the low-cut and the compression of the "bus" will print to the mixdown tracks.
That's all I can think of so far. I think once I've got some more experience I'll do a stand alone how-to FAQ.