One thing I would like to note that JAL's JGC and ANA's SFC seem to give residents in Japan with co-branded credit cards more ways to keep their status almost indefinitely once they reached threshold.
At least for JGC, I know it is possible to reach OneWorld (OW) Emerald and Sapphire with some domestic mileage runs.
The program has become increasingly popular even among Taiwanese travellers here in Taiwan. Many just spent a few days working on those MR flights in Japan and get their status. Their is a theory that the status will almost never expire. I won't go into the details here.

I just want to point out that, since you are based in Japan, and if you would also have some domestic travels, for leisure or for business, maybe JGC or SFC could be an option, too?

P.S Air China and China Eastern seems to have quite a few discounted Business Class fare from Japan to Europe from time to time. I also found that ICN usually has good Business Class deals to Europe.