>If you have start again, what would be core of building saas that makes $5k per month?

1. Why $5k?

2. Find a market and customers you can reach who have a problem they are aware of, have spent money to solve, but are still unsatisfied.

>Remember, most niche are already saturated. Community building and social posting does not yield to immediate revenue.

Questionable premise.

>What would you do?

Same as always: find customers in a worthwhile, growing, market with a problem they're aware of and still dissatisfied with existing solutions.