I've seen a bunch of high-paying dev jobs on Forkwell Jobs, and not all of them sounded that demanding in terms of requirements. A lot of the companies seem to be trying to imitate the non-slave-like conditions and benefits of Western tech companies too, which is good. There are also well-paid freelance/temp gigs on Levtech.
Whether or not a high level of Japanese is an issue at all depends entirely on the job, but of course it's a big advantage. I think the bigger issue is the visa, as already mentioned. You'll have to get yourself hired and sponsored by a tech company either way, so maybe you could buy some time by getting yourself a humanities visa with an eikaiwa while still on your tourist visa.
Whether or not a high level of Japanese is an issue at all depends entirely on the job, but of course it's a big advantage. I think the bigger issue is the visa, as already mentioned. You'll have to get yourself hired and sponsored by a tech company either way, so maybe you could buy some time by getting yourself a humanities visa with an eikaiwa while still on your tourist visa.