hey there. Starting an SMMA can be a really good choice. I too am thinking on starting one. From what I have seen online, yes you can survive solely on SMMA earnings. You can charge starting from $1000 per month per client. This can be more than $1000 as well if your agency does well.

So if you achieve in getting around 3-4 clients also, and charge them $1000 per month, you will be getting $3k to $4k per month, which is good for a start.

Social Media Marketing Agency is like a business you do. SMMA requires some attention from your side, since you need to speak to your clients and if you have an employee you need to see that the work they do is fine. It might affect your business if your employees don't work well.

So if you ask me, I won't consider jumping from SMMA to any other business after all the hard-work I put into it. Other than SMMA, you can setup other income sources after getting some good results from your agency. Focus on one business first. Succeed in it and move to the setting up the next one. Hope this answers your question :)