I've wondered about such things, and it feels like the 17 Lands dataset might be a good place to scrape play-by-play game data between human players. Feels like it could be adapted to a format usable by this structure, and used as a fine-tuning dataset.
Ryan Saxe did exactly this a number of years ago: https://github.com/RyanSaxe/mtg
Oh, fascinating - I didn't realize they released actual replay data publicly. It doesn't look like it's quite as rich as I'd like, though - it only captures one row per turn, so I don't think you can deduce things like targeting, the order in which spells are cast, etc.
(I also thought about pointing it at my personal game logs, but unfortunately there aren't that many, because I'm too busy writing analysis tools to actually play the game.)