Upwork is a terrible idea. Everyone there is broke. They're struggling to find clients themselves, and have no money left over after they possibly keep their heads above water to invest in your services.

Find a better class of freelancer for your clientele.

I don't think the startup/VC people are going to care, even though your idea makes some sense (in that helping their fundees market better = more customers = faster payback/growth), because people don't connect the dots.

You should niche down, not be 'for everyone', and focus on one kind of customer for now. Eg. Ghostwriters. You can always branch out later. But to keep yourself sane, it's a good idea to learn one niche and work it over and over again, quickly re-using what you've learned, rather than having to slowly learn a new niche with every new customer.

Good marketers do this: a bizdev pro who helps plumbers uses their experience about that industry repeatedly, getting through the projects with ease, rather than the grinding of gears trying to learn, say, the landscaping, then the homebuilding, then the paving industries.

I haven't seen your financial info but I doubt you're in a position to go after paid advertising yet. You need at least US $3000 to literally throw away and not be crippled by the loss of in order to be able to risk on paid ads and 'figure it out' as you go.

Pick a niche, market yourself as the clear and obvious choice to help them get more and larger clients, and repeat, repeat, repeat. Do not reinvent the wheel. Do not look for 'challenges'. Look for work that is easy for you. Otherwise, you will exhaust and frustrate yourself.

Might be time to go get a Dan S. Kennedy book on your marketing plan...if you have Kindle Unlimited, you can get it for $0.

This, by the way, is the #1 thing I see missing from service providers and their marketing plan: FOCUS.