iPhone sends multiple text messages

When I send a message from my iPhone 12 mini, the recipient gets 4 messages in a row.

I've tried fixes as detailed here and here, but since the iOS settings keep changing I have to adapt the suggestions to the current iOS, e.g. "forced restart" means following instructions here, being told to "turn off iMessage in settings" is meaningless because there is no iMessage in settings.

There are even youtube videos like this that take nearly 3 minutes (plus ads) to say "turn iMessage off and back on again".

I've turned off repeat alerts, I've asked recipients to turn of repeat alerts, etc. but nothing fixes this issue.

This topic seems to go back many years, yet there doesn't seem to be any fix. Certainly no one has replied to any of the advice posts with "hey, that worked" or similar.

Can someone please advise a current fix? Not something that is 2 or 3 years old because those fixes discuss settings that don't exist in the current iOS. Why Apple keeps changing settings is beyond me.