Hi there! Off the top of my head, I can't remember if, when I used Textra, the messages would pop up and sound over my Google Map, even if sound was enabled.
If you have all of the appropriate settings turned on and still no dice, I'd contact Textra's app development team. They respond very quickly and are always helpful. Please let us know what you've come up with, as a lot of users here have Textra as their main SMS app.
I use an app called Audify that works great. You can set it up to read any app's notifications that you like. It gives you the option to only read your chosen app's notifications outload when connected to a charger, a specific bluetooth receiver, wired headphones or always. Also has a quick settings button to stop reading temporarily if you need. Great little app.