Thanks for your quick reply, but I still don't think that the way you argue fits the reality for most indie game devs. Usually people don't invest so much money just to prototype a single options menu. Seriously, which gamedev invests 25€ just to prototype an options menu for potentially 4 different themed games? If you look at gamedevs like Luca Galante, the developer of Vampire Survivors, then he didn't use your assets just to prototype a game, did he? Even if a gamedev wants to prototype just an options menu, in this case it is 99% of all the time just for learning purposes, where the technical functional programming aspect is much more important, then the actual design. Almost every game engine delivers a basic UI kit, whether it is a slider or a button.
But this is not important as I don't want to start an argue with you, whether your assets are mainly used for prototyping or not. I don't know if this is the use-case for all of your customers. However, I am still very much convinced, that it is way better and customer-friendly to pack all of your 4 GUI themes into separate single bundles, in which a customer can then freely choose a full set of a specific themed bundle to build his game.
Just an example:
Let's say I want to make a medieval game and I am looking for a really nice stone GUI. Now here is the problem of your stone GUI:
1) My fictional game has an options menu, so I need to buy "Option Menu Designer Kit" for 25$, where 75% of all the assets included I don't need, because I just need the stone GUI.
2) My fictional game also has an inventory, so I need to buy "Pixel Art - Inventory Designer Kit" for another 40$, where 75% of all the assets included I don't need, because I just need the stone GUI.
3) My fictional game uses skill bars, so I need to buy "Pixel Art - HUD Designer Kit" for another 15$, where 75% of all the assets included I don't need, because I just need the stone GUI.
So in conclusion, I need to invest 80$ to buy three different asset bundles just to get a full stone GUI for my fictional game. As I already said this is inefficient and unneccessary, where I could instead buy something like a "Pixel Art - Designer Kit Stone Theme" for 40$ instead, which is much more attractive for me to buy and you would have won me over as a new customer.
Sure, a commision is still more expensive than spending 80$, but at least the assets are owned by the gamedev's company and your game stands out. So the higher price of a commision is not really an argument, because a gamedev pays for more than just the assets.
This is just my individual feedback coming from a gamedev, not an asset publisher. :)