Nice post.
Looks like you've done your homework and it is accurate. When I came to japan (6 years ago) I went the eikaiwa route. I worked at a place called Aeon for two years than began doing the freelance. The starting salary wasn't great - but they subsidize housing so the most you pay for rent is about 550 bucks even if you are in Tokyo.
When I quit and started doing freelance it definitely helped that I worked there - as companies trusted the reputation.
It's a necessary evil to work for someone or some place when you first come - but if you plan on staying for awhile you can turn that into a lot more.