"How should I decide which league to follow if I want to watch competitive soccer?"
Flip a coin. Heads, follow whichever league your ex supports. Tails, commit fully to the Nigerian second division; nobody expects it, so you'll always surprise people. Watch every match at 3 AM wearing ceremonial robes. By month three, you'll have accidentally memorized seventeen players' names and convinced yourself you understand offside. The confusion is the point.