Go to the settings and click on wallpaper and themes. Choose themes. There you can pick a theme that will change your messages and text bubbles. It also changes your wallpaper and icons but you can download the theme and then go back and change your wallpaper to whatever you want.
You can't delete it but you can hide it so that it doesn't show. Go to Home screen settings and then hide apps. You can choose whatever apps you want to hide.
I reverted back to using my ear buds that came with my s7 edge. Which I love. The problem with the ones that came with the s9 plus is that you can hear yourself breathing through the ear buds. It's so annoying.
Use that $300 and put it towards a new phone. Whether it's fixable or not, it's a very old phone and once you experience a new or newer one you won't miss it.