I find a better approach is to focus on a few basic ideas you need specifically for your work and digging deep in there. Nobody can be expert-level in everything, but you can be expert level in your specific domain of research.
Also for ML stuff, it's hard to overemphasize the importance of understanding linear algebra really well. Here is a excerpt of a book I wrote on LA which should get you started on learning this fascinating swiss-army-knife-like subject: https://minireference.com/static/excerpts/noBSguide2LA_previ...