iMessage for Business is very restrictive and has a really long approval process. On top of this, it also sends gray bubbles and doesn't allow any outbound, which prevents consented outbound use cases such as form fill text back.
> also sends gray bubbles
Incoming messages are _always_ gray on iOS, irrespective of the use of iMessage or SMS. Your solution is not any different in this regard.
> iMessage for Business is very restrictive and has a really long approval process
For good reason, so companies don't abuse it.