With that in mind, what kind of profit are writers/designers making the way you outline, forgoing the connection to WotC IP?
This is a good question, but hard to answer. Perhaps the most true answer is that just about no one is making money of consequence. And just about no one can, through any model, cover the costs of a mortgage, kids going to college, retirement, and health care later in life. This is an industry that functions more like a hobby even for professionals. It is seldom one where your products can pay for your actual cost of living. There are exceptions, but those people are kind of special for complex reasons.
To get a bit more to what you are asking, profit is potentially much higher if you publish elsewhere for several reasons. One is that you get that 50% back and get to keep it
if you can draw sales. That's the big if. I'll give an example. In some circles, I am relatively well known. However, my sales on both the DMsGuild and DriveThru are just okay. I could never cover my cost of living through them. However, when I wrote
Forge of Foes with Mike Shea and Scott Gray, we were able to add our DriveThru products as add-ons. Mike has a tremendous audience, far beyond what I do. Before the crowdfund, my two DriveThru products had about 100 sales each. Via this Kickstarter, I sold ten times that. This was only possible because they are on DriveThru (edit:
and it only mattered because of the audience Mike can draw). I can't sell my DMsGuild products anywhere else.
I hope the complexity is clear here. There is no simple answer to your question. On my own, having DriveThru products doesn't really help me greatly. I offer them on my Patreon, on the Mastering Dungeons podcast Patreon, on my Ko-fi page, and I mention them on Alphastream.org and on my videos. I get meager sales from that. But through a SlyFlourish Kickstarter? That's suddenly a huge audience and the profit is relatively huge to my normal profit.
That's the thing. It depends on each creator. If you will have 20 sales, the platform doesn't matter. The more you have an audience, the more you can turn that profit dial way up by being able to sell in a variety of places.
Just as an idle thought, is there any point in doing "loss leader" products on DMsG and use those as advertisements for your main body of work over at DTRPG? As in, "Here's my Caves of Chaos tie-in adventure. If you like that, go that-a-way for more."
Generally, no. You will generally be better off by drawing audiences in other ways. This is in part to the platform not really allowing you to link to other places where you sell. Some companies, such as Evil Hat, have really refined the technique of free products on DriveThru that drive purchases, but it takes some real diligence and experimenting to pull that off well. Jeff Stevens has had success on the DMsGuild with a free preview of a magic item, which showcases his magic items paid product - subscribe to his newsletter to hear about his experiments. As with the first question above, the answer is complex and depends on your audience and what you can do to leverage sales. For most folks, the sales aren't substantial either way and it is best to look at the hobby as a fun place to be, side money to cover gaming expenses, etc.