Wow. You must have a brilliant mind to have come up with this - and just for this jam too! A lot of thought have gone into this as it's like creating a chess game with your very own sprites and rules in such a short time! Full of respect and admiration, man! After reading some of the previous comments and your replies, here's my thoughts, that I think may be able to help you :) :
There is definitely a learning curve for this game which is inevitable, and I would say it does take time to learn so I can see your challenge trying to convey this to the player - I myself am not v familiar the the rules of chess tbh so you can take it that I have 0 background knowledge and making reference to the rules of chess may not be 100% effective. As such, I'd think this game can be quite advanced and complex for the average casual gamer/public (assuming this is your target audience).
Whilst I think the detailed text explanation is helpful and important... may I suggest that instead of removing it or shortening it *too* much, instead, add some animations/gifs to show the sequence of the turn-based, what action should be done per turn, and how they (both the player and enemy) move/attack just beside where you are explaining the in the text can help with conveying this? Like one section/para can have the corresponding animation/gif beside it . This will really help with visualization and understanding.
Also, perhaps start with an easy level with maybe just one (or two different types if you feel is important for the player to know) and with prompts on what to do next (like a model answer). And break it down explicitly by steps within each turn. In this way, the player can quickly get the hang of it and be able to continue the next levels themselves independently without losing interest before that stage. You can also add this to the "How to Play" as part of the tutorial / something the player can go back to refer if needed.
Hope the above helps and all the best for your game! I like it, its looking good! :D