I've learned the most from implementing papers. And being stuck. But me is me. Since you mention SE, I'd choose a mini project in an area you love. The tooling

I've learned the most from implementing papers. And being stuck. But me is me. Since you mention SE, I'd choose a mini project in an area you love. The tooling you will learn along the way. An hour a day is paradoxically not nearly enough, yet also a serious time investment of your day. Maybe start by asking what exactly you want to learn? Applying ML to a practical problem, in user app? The math? The ideas?