Do you have any idea why they are giving you only 1 year renewals? Previously on an instructor visa I had no trouble getting a 3 year renewal -- even though I only had 1 year contracts at my employer. I wonder if it's because they see your employer as being risky (given that it's a startup). You might consider trying to get a job somewhere bigger (or trying to get a full time position) if you want to stay in Japan forever.