(Gameshare) my son and I are going to game share. For future purchases which account should make purchase?

I just recently bought a ps4 pro. I know to gameshare from my sons ps4 I need to set his account as primary on my ps4.

1. Do I need to set his ps4 as my primary as well? So his is mine and mine is his? Or are we good with just his account primary on my ps4 and his set as secondary?

2. For any future purchases which ps4 do I make the purchase with in order for us to both have them?

I've searched all of reddit and online but haven't found a clear answer for #1 and nothing at all about #2.

Thanks for the help.