I don't know your exact area of expertise, but the salaries for software engineering in Tokyo tend to range between 4M for a junior to 12M for a senior. Some big companies (especially in finance) may go as far as ~15M, but it's pretty exceptional.

Unless you have a specifically rare expertise, I do not think it is realistic to ask 20M, it will probably make the company reject your profile without negociation.

Now if you go the freelance route, it's probably feasible (but not necessarily easy, it depends a lot on your Japanese level and how much you want to work and can handle context switching), but you'd need a plan to get an appropriate visa first.

Source: I have been living here for 5 years, AMA.