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.
“Consented outbound use cases”
This is top-of-the-line corporate jargon.
> 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.