I have not bet on sports betting other than the odd bet from time to time but I have been interested reading articles and watching interviews and what I usually read from successful bettors is that they try to find a niche in the less popular leagues.
I don't know where you learnt this stuff from, but this is actually a tough path to earning big in the sports betting world. Why? Because sportsbooks tend to limit gamblers who consistently win by betting on lower level leagues.
They tend to misprice the odds from lower level leagues quite often which some clever punters take advantage of, but only for a short while.