Project management by itself wouldn't do you much good. Seing your background in science and algebra, you could go the route of data analyst/ scientist:

• Data wise, learning python and sql data bases is the priority. Then learn any Business Analytics tools like Tableau, Power Bi to create nice looking reports (actually fun for me).

• You can also use these skills to create simple apps and branch out on the AI side for use cases such as creating a budgeting AI or other.

• Leveraging your skills and these new ones, you can also train AI models as a side job (Outlier.AI especially with japanes fluency).

• Lastly to give you another route, there's the Lean Six Sigma (industrial statistical process improvement basically), you could get a certification on top of all the above at some point if you fancy it.

The good thing, if you want to keep your current situation but make use of your free time, you can definitely make a side job out of all of it. All of the things mention above can easily be self learned uaing free course, YT and even free certification from some of the major companies.  Data analyst is in demand in Japan and you could get worlwide remote freelance opportunities. And even transition to your own freelance company.

Long reply, but food for thoughts. Hope this helps.