Hiya- I've tried Apple senior tech and they just cannot fix this- I do not work for Apple. I'm a rescue volunteer with the same problem. Apple have thanked me for the work around I have suggested. Purchase a 30 second tune and set it as the text tone. It's only 99p. I've also set up a shortcut to play music on my phone whenever an important text comes in.
Apple could get a real PR boost from this if they could sort it out. Loads of users need an alarm text system. SAMSUNG really have the jump on them here. I don't think Apple know what a difference they can make to blue light workers and care workers and industry safety workers.
please please please log a complaint with Apple so they can get the message. I'm not sure they are aware of the difference they can make.