I agree with you, multiple streams of income gives you a More financial leverage as you buy aggressively, multiple streams balances the quest to invest more, because you don't have to be dilly-dallying with DCA strategy because you will be investing from a comfort zone. Multiple streams gives more flesh to both your discretionary income and your emergency funds, this is not to say that people will single source of income cannot do all of these but multiple stream places you at a more advantageous position.
Yes you are right but if you find a second source of income and work to invest it is very good. But you have to take care that you do not harm yourself while doing this work. For example, many times our health gets very bad while doing a lot of work. And later we become very sick, work by considering all these aspects. Remember one thing, the richer you are, the more you spend. So if you can do extra work then continue working if you cannot then change your decision. Because you want to do this to invest extra but if you get into a big physical problem then your investment may be at risk.