It is hard to find part time sales/biz dev roles from larger companies. By the time they are looking to hire, they are looking for a full time person because they have a (mostly) full time need. Especially if there is commission involved, then it just continues to feed into the more, more, more...

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?