Gotcha, interesting. It’s your game obviously, but if it were mine, I would just use a fixed aspect ratio, that way the gameplay would be the same for everyone. 16:9 seems like a good middle ratio between 4:3 and 16:7. You’re still using 90% of the area for gameplay, and you could use the black bars to put GUI stuff out of the way.
A fixed width and variable height probably wouldn’t make more work for you, but then it would be harder (less time to see what’s coming) on fatter screens and easier on taller ones. (You might scale down the vertical speed of everything to even it out somewhat.)
With a fixed height and variable width, you would probably want to scale your enemy movement patterns, otherwise the player could always hide out at the edges on wider screens. And the gameplay would still be a bit different, you might want to make the player faster, since they would have more horizontal space to cover.
You could try fixed area. It’s a middle-ground and might work out pretty nicely, or you might just have to deal with all of the above problems.
Those are the only four options that I can see. If you don’t use a fixed aspect ratio, then either the width or the height or both are going to change. Using an aspect ratio range is not a bad idea, but it’s really just lessening the effect, you will still have to deal with slightly different width/height.