Can someone please remind me of the best date/stage to transfer a '22 save to the new game?

I believe it May 1st. If you click on the import in DDSCB23 it will give you all the info there.

Yes it is especially important that you transfer the save in on May 1st because the "buy reports" stage is no longer present in the game and it is replaced with the budgeting stage on that date

There are instructions on the screen that opens when you select to import..be sure to follow those carefully. Be sure to SAVE the association immediately after it's imported.