This is just common sense, I would never understand people that want to apply in another country and expect to do it exaclty in the timeframe suggested by the embassy. Stay 2 weeks and have some margin, even if the embassy says 5 days, why take the risk of having to leave before you get it, move the flight or some other stuff?
I'm a freelancer too, I have my trading licence registered here in Czechia and contracts with my clients (required by the Embassy, contracts both signed by me and the client).
I didn't have the contracts and I had to create them and ask the clients to sign.
Portfolio is often required (depends what kind of freelancer you are), since I'm an IT consultant I don't a portfolio of apps or works to show, but contracts with my clients for sure.
If you don't have a portfolio, contracts with customers or any product from you company, I seriously doubt you will get it following the remote worker route.
So or you wait to have customers to show contracts with them at least, or you take the soft power route, there's not much else to do.
I think a cover letter explaining everything should do the trick, I prepared one since I will submit 4 bank accounts but I will also explain there's no portfolio as they expect, not all freelancers have products they sell or make, there are services that we provide and it could be silly to make a list of companies we worked with but maybe also that could be something to mention in the cover letter, supported with invoices to show you worked for those companies (I will do that also, list of clients with invoices).