What did you change exactly, the text or the image itself? I got stuck at around 600 subs last year and what finally worked for me was making the text on my thumbnails way bigger, like 3 or 4 words max in a bold font. People watch on their phones and if they can't read it at a glance they just scroll past. I also started testing two versions of every thumbnail before posting, just letting them run a day apart to see which one got more clicks. Small stuff like that adds up fast, sounds like you found your version of it.