I really love this idea! I've also messed around with a similar mechanic in GDevelop after thinking about how much I enjoyed games like DungeonRaid and Card Crawl on iOS but never completed it. Now I feel encouraged to come back to that prototype again and tinker around.
You did a great job explaining the mechanics in the beginning and the instructions on the page are clear as well. Your UI is also easy to read and not intrusive. Overall just a completely solid gameplay loop, nice sprites, and a good challenge. Impressive work!