Your project looks really cool, but unfortunately I'd be looking to limit distribution of the software for profit. GPL seems like it'd be the most appropriate, but I'm by no means an expert on this stuff.
It might just be overthinking on my part, but it'd be a shame to somehow negatively affect the Windows customization community. There's so many paid desktop customization tools out there for Windows and many of the most prominent ricing tools are closed source.
Releasing the source code under GPL implies that if all or part of the code is taken, integrated into another application, and that application is redistributed, then the source code of the application must also be made available under GPL.
Note that:
1. GPL is not distinguishing personal and commercial. However, those selling applications may not want to release their code. Hence, they may not want to integrate GPL code into their application.
2. If someone compiles your code into a binary (or just gets it from you), they may use it for both commercial or personal purposes. GPL is not coming into action here as it was applied to the source code, which hasn't been modified in this case.
3. GPL clicks on redistribution. So if someone has integrated GPL code into their application, however, are using that application for themselves only, or within their company (which is considered a single entity; distribution within a company would not qualify as redistribution), GPL does not come into play. Likewise (not applicable to your case), if someone is using the application only on their servers in the cloud and only serving output to the customers, they are not redistributing the application itself. This, in a manner of speaking, defeats the purpose of GPL in the cloud world, and is sometimes called the server loophole. Alternative licenses like AGPL are more suited against this.
4. If what you are truly looking to do is to make money by selling your application, you may consider not releasing the code openly at all (you can still delete the code if you like prior to attaching an explicit license; once released under GPL or another license, someone can legally copy before you take back). There are alternative ways where dual licensing is used, etc.
I did not understand your second paragraph to be able to comment much. It seems that you are worried that your releasing this as free (or FOSS) would negatively impact sales of similar existing third-party commercial applications.
If I understood correctly (else, please ignore the rest of my comment), while of course the choice is yours, this is a common scenario. Various producers are to be in a healthy competition to serve the consumers. So if a producer can provide the same value to the consumers for less, that would be considered improved economic efficiency.
The challenge usually is that lower quality stuff shows up in the market for cheaper, impacting the market for better quality higher price. This itself is also OK as such, for if the quality is lowered too much, the consumers may naturally prefer the costlier product. However, challenges still result for various reasons.
I should've clarified to mean the open-source Windows community in my earlier comment.