Looking at various options for a oneway economy flight from Europe to Japan in early September.

It is a bit of a toss-up at this point.

I normally use the miles for a J seats as there is much better value there, so may end up paying cash. Of course sitting on miles for too long is also not a great idea.

Taxes/fees on some of these are out of hand. United is low but mileage requirements high:

Hainan ¥65,000
Etihad ¥100,000

or with mileage:

Korean (Peak Period unfortunately) 52,500+¥40,000 (J 92,500+¥56,000)
United 60,000 + ¥10,000 (J 140,000). I think a couple months ago it was 40,000 for econ and 80,000 for J.
Delta 50,000 +¥34,000
Aegean 45,000+¥48,000
Alaska 37,500 + ¥16,000 (I bought the miles for around 2cents)
American 35,000+¥7,500 (J 75,000 but I would have to buy 100,000 miles to cover 2 passengers)