I'll get this conversation started. I love shows like How I Built This and Tim Ferriss' show, but too often the people they interview are so experienced that the gloss over the "how" part, and I always feel like I want the meaty substantive parts. But I get that it's difficult to do when you're attempting to tell someone's entire life story in 45-1hr.
A few things: Startup founders need to be convinced that you'll be willing to do the work that at big companies you might have had a support team for. So you need to prove that nothing is beneath you. The best way to get their attention is to demonstrate the type of work you'd do for them right in your cover letter/email. (ex) In your case it could be sending a recording of how you might lead a technical demonstration for their product even if it's just a mock demo.
Also, I'm not sure how early stage you're willing to go (looks like you want at least 10-20 employees), but with your experience you can fairly easily start to engage startup founders that are just starting to think about building out a sales team and process, and help guide them. Whether it's tips on how to create pitch documents, how to set meetings, or the best way to think about how they will scale their sales team. I've seen a number of startups where someone has started off as an advisor (whether formal or informal) and later on was hired as a FT employee. This approach might take some time, but at the very least you'll build relationships with interesting companies and get to see how they're run before joining them.
I think you can start now. If you've built enough to have a decent portfolio to show, then you should be able to get clients based on that alone. Team up with a few people that can make up for any weaknesses you might have and get to it. IMO design skills are huge since the way something looks is often how clients will judge your work, so find someone who can wireframe and mockup solutions quickly and get your initial clients excited about your work. You can start with smaller projects and build your way up.