JAL has very limited first class seats when they are first available, and a handful of business.
It’s decently easy to find around 2 seats in business here and there at any date, just depends on the route/load.
The key is two weeks before a flight, at that point JAL opens up a majority of First and Biz seating for awards, so at the two week point, that's when you will see it.
As others said it’s best to just book what you need, then when you see availability at the two week or less mark, call up and change.