Here's my first variety puzzle! I think the special rule is really interesting, and though final hidden message might be a slightly niche reference it is not required for solving the puzzle and is just a bonus. Hope you enjoy!
Small hint: The solutions to the thematic clues are all longer than their grid entry.
Medium hint: The transformations in the thematic clues condense a portion of the solution into one cell. The rebus function is not required for this puzzle.
Large hint: Ten cells do not contain a letter.