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.
For good reason, so companies don't abuse it.
This is top-of-the-line corporate jargon.
Incoming messages are _always_ gray on iOS, irrespective of the use of iMessage or SMS. Your solution is not any different in this regard.
As far as I can tell, the OP’s insistence that their service won’t send gray messages seems entirely disconnected from reality.
Personally I don’t see why you’d care. My business isn’t trying to pretend to be a normal person using a phone, so why would it matter?
https://register.apple.com/messages
Looking at their approval process, it is involved because Apple is focused on their customers, who are the end-users. They want it to be "great." Just letting any random company connect to their API and blast Apple users isn't going to fly. Users would hate it, and it would destroy their trust in the channel. Spammers and scammers ruined SMS. Nobody wants that to happen to the other channels too, especially not Apple.