One thing you might not have considered is that, this way, the chaeto and ulva will get MUCH less flow, which would (theoretically) greatly decrease their capacity to remove nutrients. In turn, that would decrease their growth, which would decrease your potential profits.
Also, when you keep it AIO, you make it more difficult for yourself if, in the future, one or more of your fish starts to bully the others/is a victim of mass bullying. If you have a separate refugium, then in that situation, you can easily separate out the problem fish or victim fish by putting them in the refugium until you can either sell them or set up a separate tank for them.
External refugiums have another use, too: you can easily put your trimmings from the DT in them and propagate your (display quality) macro algae to sell online.
Lastly, consider the plankton. The overflow for your tank may be large, but it is small compared to the size of the external refugium. I don't know how large you were planning on making that refugium, but I suspect it is at least 10g larger than the overflow, and probably a whole lot more. This decrease in volume means that the amount of plankton you can potentially produce is exponentially decreased! And while you might consider it a benefit to have plankton picked off through predation with no effort on your part, it makes it more difficult to know how much your fish have been eating and whether or not they have been eating at all. Not only that, but it makes it much easier to over and underfeed your fish with store-bought foods. Underfeeding is bad for obvious reasons, and overfeeding just might be worse, because that can easily wreak havoc on your water chemistry and potentially cause an algae bloom or, at its most drastic, crash your system.
...Oh, and it means you will lose flow in your DT, because that is one less pump. I imagine you have your pumping scheme all figured out, and having to change it to compensate for the loss of flow could be a headache-inducing problem. So, for the above reasons (and probably some others that I haven't thought of yet), I suggest that you don't scrap the external 'fuge.
On a different note, I was just watching Jamie Cragg's MACNA talk, and he mentioned that they use yeast to feed their bacteria, which I thought you might be interested to know. And if you haven't already watched the recording of his speech, I highly suggest you do so ASAP! It is a really fantastic presentation, and it was so interesting to learn more about the work they are doing at Project Coral.
