My question is when to do account migration vs cross save. I have two accounts. One is on Battlenet and the other on PS4. My main account is PS4. Do I need to wait and do cross save over to battlenet on the 21 and then migrate my account? Or is is better to migrate my account and then do cross save? I have all annual passes and DLC on both accounts. I just want everything to be playable.
The order doesn't matter. Cross-save takes care of your characters, their gear and stats, whereas the migration is just moving PC stuff (content entitlements) from Battle.net to Steam. If you set your PS4 as your cross-save account, then those characters (and their gear etc) will be used to play on PC account. What content you own on a particular platform determines what you can access etc, not the origin of the cross-save characters. Since you own all content on both platforms, you just link and migrate (or migrate and link) and you're good to go.