It's the Bank of Japan who decide the borrowing rates, so, not problem in that regard.

Ok, but they don't control inflation. At some point, I think it's obvious that printing money becomes counterproductive. But where exactly is that line?

The Bank of Japan, can actually control inflation, because, as we have determined, they control the interest rate. Also, if necessary, the government could raise taxes and run a surplus to diminish inflationary pressures.

But those are moot points, because the problem of Japan for the last 20 years has been deflation, not inflation. Despise that, as you can see in the comments here, there are people that still predict inflation in the future.

They don’t control inflation. But all of the warnings about hyperinflation being “right around the corner” have remained hilariously false. So who knows?

They only control their inefficiency around an increasing trajectory