A degree , maybe a part time professional masters perhaps from a school where the faculty who teaches does active research. Or just read papers. 2 min papers is a good channel to start off with.

In ML/DL 1 year is already ancient. 4 years is prehistoric lol. For context if u choose a topic, say 2D-3D translation , from the advent of NERFs a couple of years back? We have a stupid amount of papers on the topic trying various novel approaches , everything ranging from using Voxels to store geometry in new ways, to geometry aware GANs , multi view compression using ViTs etc etc

So choose a topic and focus on that otherwise it’s a lot.