Boy this a tough one to answer because it changes a lot over stage and time.
Let me answer a simplified version of the question for now, around your first 1–3 CSMs.
You can get away with a lot here. As long as they understand your product and are insanely committed to your customers.
That will already be 50x better than having no CSM coverage on customers at all.
So for your first few, you can hack it. They should have some on-point experience. But it doesn’t have to perfect, as long as they meet these criteria.
Later, things will change. You’ll have more process, more technical support, more revenue commits to the team, upsell goals, and much more. The skills will become more specialized and professionalized.
But at first, passion for the product and customer commitment, together with decent organizational skills, can take you pretty far.