Any suggestions?
Not sure where you are located, but in the USA this is possible.
An alternative would be to get a full time job, but make sure there is a carve out for work you do on your own that is unrelated to the job, and work nights/weekends.
Final alternative is to work full time and just save save save, then quit and work while living on savings.
Not sure of your financial situation and obligations, so hard to recommend any of these above the others.
Source: Before moving into consulting, I ran sales teams.
I would imagine that Customer Success would be easier to find on a less than full-time basis.
Consulting as Mooreds pointed out could be a good solution. Could you consult or do paid work for clients within the realm of what your start-up is going to be doing?