I have exactly the same problem where sharing from any source (Photos, Twitter, YouTube etc) defaults to share via WA Business instead of WA.
I have followed the instructions given but this does not work. Even with WA Business removed (greyed out) it still comes up by default.
I think it may be a bug because I can get it to work if I hit the three dots, Edit (no need actually to do anything as WA Business is already greyed out) and then share via WA. But then next time, event though WA Business is greyed out, it defaults to it.
It's almost like it needs "reminding" that it is greyed out by hitting Edit each time before sharing.