Some players have said that you can’t unlock everything in one playthrough.
Yes, that is by design. I want players to discover new things in future playthroughs, so the game still feels fresh even after you have already finished it once.
On the other hand, I also feel that players who have completed the game multiple times should get something in return.
So in the next version, 6.6.3, if you achieve the BEST Ending S, you will be able to choose to stay in the game after the ending. This extra period will last from January 1st to August 31st — the second-last day of summer. That is one full calendar year including both normal & after-ending period.
There won’t be a lot of new content in the after-ending period, for now (pretty sure I'll be adding more stuff later). It is mainly intended as 8 months of extra time for players to collect, explore, and unlock every little secret they may have missed.