4GB of DDR4 is a lot worse than 8GB of DDR3. Those (slightly) older business SFF computers are plenty capable compared to the pi and their software support is at least as strong.
You're also going to have to add several peripherals to the pi that aren't included in the price.
Some percentage of revenue for using other people's IP is pretty normal.
And I think it's hard to argue Baldur's Gate and using DnD isn't a meaningful part of its success. Divinity Original Sin 2 is a really good game with a lot of the same DNA (it's why I personally bought BG3), and it stayed pretty niche. The IP is a big part of it exploding.