Ah, I managed to transfer my save files! All I needed to do was copy the compressed zip folder that I installed from itch.io and then paste it to the Steam file. Now I can continue my previous save. Just posting in case anyone else is wondering if it's possible - it is :) Also, I didn't have to obtain my achievements again. As soon as I pasted the folder and it finished extracting, my achievements were credited :) Yay!