Welcome to HBGames, a leading amateur game development forum and Discord server. All are welcome, and amongst our ranks you will find experts in their field from all aspects of video game design and development.
You'll need a script that rewrites the Tilemap class. I suggest you use Seph's or Poccil's script, there's already a variable for autotiles speed in both scripts.
Yeah, autotiles don't have to run for only 3 or 4 frames. I haven't tested any more than 6, but I know that it'll keep animating if you keep adding 3-tile-wide autotile chunks in a row.
I thought about longer autotiles but that will take up too much space if I do it a lot. ^^; Thx for the tip tho.~~
Edit: I tried the script from here (Poccil's), but it just keeps adding the autotile on top of itself, which causes semi-transparent parts to become completely non transparent. X_x