I had a look at Sieve. I like your vision and I'm eager to see what you'll be to able to contribute to make that vision a reality.
For my business in particular, such an all-in-one product doesn't make sense because I already have tools and processes in place for covering the various aspects of the business:
- website
- proposal management
- requirements engineering
- accounting
- CRM
- document management
- process automation (via Zapier, mostly)
I like to see this as a more UNIX kind of approach with specific tools for specific jobs that communicate with each other if needed. Even if I were to forego this approach in favour of a single tool or product that product would have to be better by at least an order of magnitude compared to my current setup. For what essentially still amounts to a process and management tool that's quite a tall order.
Freelancing / consulting isn't a one-size-fits-all, cookie cutter kind of business. Therefore, a unified product that tries to cover every aspect of the business probably isn't going to cut it, at least for those who've already been in the business for some time. However, they're likely also the most interesting audience for a product such as yours because they tend to have more revenue than businesses that just start out.
There's an admittedly - and unfortunately - quite large group of freelancers though your product might be very useful to: Those that don't do any kind of marketing (i.e. they often don't even have a website) and mostly rely on manual processes and middlemen to do their marketing for them.