I think your chances of being approved for this visa type just on the basis of being a freelancer are slim, particularly if you have no Japanese clients lined up. If you do succeed, you'll likely be required to form a company structure, pay yourself a salary out of that structure, and file taxes for your company as well as for yourself. There are also lots of gotchas where you can end up, for instance, having to pay corporate taxes on the 5 mil you sunk into your own company.
If you are ok with settling down for the duration, you're much better off finding an IT job working for an existing company in Japan. For IT, you will likely be granted a 5 year visa without the need to show 5 million yen. Your employer also brings their creditworthiness to the table and this makes finding an apartment much easier. It's a much better deal than the startup visa, if you can accept the 9-5 office job aspect. When I lived in Japan, there were foreign companies that operated internally in English. They had trouble finding English speaking IT/SW Engineers in Japan, which made it a viable option for expats.
If things don't work out, your visa does not terminate - you're able to stay in Japan and take additional jobs within the field (so staying with IT work). Those new jobs could be contract work for Japanese companies. Or just keep "looking" for 4 years.
I lived there for 10 years, but as an employee not a DN. I had one friend do a startup. Feel free to PM me if you have any questions.