Letterbox scale isn't ideal when catering for different screen sizes on mobile, as even though you can change the colour of the bars, its limited to one colour. So if your game has multiple different backgrounds, you'll end up having bars just in a different colour.
Try full screen mode: scale outer and set your backgrounds around 20% larger than your viewport. This means you can have a range of different backgrounds that'll fill the whole screen
Here's an example of how it looks and displays on a 20:9 device/1080x2400 (pixel 6)
On a regular 16:9 device the "fire rate" bar and buttons would be perfectly level with the bottom of the screen but because the aspect ratio on this device is so much longer, I've extended the background to cover - just have to make sure the background looks right with the UI
Edit: blackbar at the top varies from different devices - this is because when exporting I opted not to have notification bar hidden. This helps with phones that have the camera in the screen rather than above it, none of the game is hidden by the camera. On phones where the camera is not in the screen, you would see normal notification bar with time/notifications. Pixel 6 requires you to pull down from the top to see the time when a game is running FS