Edit: I spent a long time writing this, many of my points have been made already
I have no problem paying $20 for the game, I just feel as though what's being offered and the way you're being upsold is tainting the deal.
I think a better compromise would be something that is purely aesthetic rather than something that does affect how the game is played. A good example would be 'classic tileset mode' or something similar.
The problem I have with the current set up is that even if the intention was to allow people to pay more if they wished, what you have done instead is bribe people into buying a 'complete' version instead. This could possibly be resolved simply by changing the description of the version. make people aware that the extra content will be lacking but is just a small way to say thank you, don't just sell it as the collector's edition.
As I said in my earlier message, don't act like a large publisher would, use being indie as an advantage. You don't need to sound official in your descriptions of the product, explain it how it really is. When you type "Special Edition bonus content: Additional Kingdom building, extra challenge quests and a unique character class" you don't show the same informal, human emotion you do when you say "we brainstormed and saw what sorts of bonus content we could add as our way of saying thanks."
The first one makes it sound like an upsell, the second makes you sound like an indie startup who is grateful for the custom.