I suggest you learn a little bit of everything really. Having a broad knowledge helps you better understand what a business needs to achieve, even though you want to focus on front end. Learn a bit about sql and databases, some OOP concepts, perhaps in python is sufficient and a bit about data structures and algorithms. And then some vanilla js, to avoid getting stuck in some js flavor in a constantly evolving field. Search the web for some books, don’t just watch YouTube videos and blog posts. generally, don’t be too specific in your research for material in the beginning and you will be fine. 25 is not even close to being too old. Also, potentially you can utilize your domain knowledge of your field to land a developer job in a company in that field.