Turn off Swooth Sound on Outgoing Message

You have to go into settings>sounds>text tone. Change either to vibrate or none. I prefer vibrate.

Next step, while it is time consuming, so worth it. Go to contacts>edit>text tone and assign a sound. It’s only for incoming text messages.

Choose the contacts you text. This will limit the time it takes.

You wouldn’t hear the swoosh sound when you send an outgoing iMessage but you will have sound for incoming iMessages.