For the record, since I don't see it mentioned.
Itch sandboxes games so, if you are using that feature here for safety, you will need to manually copy your saves from the itch user profile's "Roaming Low" app data folder to the one for your actual login account for the game to continue your saves.
Most of my achievements (76% completion) popped instantly on loading my save, but it seems *none* of the "purity" ones do. Sad, since that's the time-consuming and somewhat annoying route to run, but you can finish the game with your existing saves but you will need to run the section with Wildfire again to have any of the "good" route that you completed in the free version count.
It also doesn't count your previous battles, so the achievement for winning fights has to be done again as well, same goes for the song one where you need to do that minigame without any misses.