My suggestion for what it's worth:
First, don't tank, keep building your team but adjust your timeline based on the new structure so that you peak at the right time.
Second, do things now that will make your team as strong as possible for Season 41 to try to win your D2 league and promote to D1 for season 42.
Good luck, it would be awesome to get as many Japan teams as possible into tha top league.