I’m not a Fly employee, so I’ll let them answer if they can make a solutions architect available to you. But my advice would be to build something with the excellent documentation now. If you can get something working, then you can always improve the design if you need to scale.
(Most tech projects fail due to a lack of money, a lack of marketing, a poor business plan, or insufficient tech skills. So you get bonus points for building it in a scrappy fashion, so you can get it in front of customers quickly, which in turn helps you work out what they’ll pay for. With that in mind, I’d say the architecture diagram you’ve linked to is too involved for a prototype. IMO, build something smaller to start with, and set up an invitations system for potential free-tier users.)
Of course, if you can set out in detail what you’re looking to achieve, then I’m sure regulars here will give you some suggestions. What features shall your product have?