I don’t know enough about tilemaps to help you with that.
You can’t use SHOW_ALL, since you no longer have control of the area where the black bars are. So, use either FIXED_HEIGHT or FIXED_WIDTH.
Let’s go over what you’ve chosen. Your selected design resolution is 480x320, so that is an aspect ratio of 3:2. That 3:2 window will fit inside bigger view areas of 4:3 and 16:9.
For 4:3, you would choose FIXED_WIDTH, so the height becomes variable, meaning you need to have a background that is the following size:
Width of 480 / 4 = 120, so multiply that by 3 for height, and you get 360. Final background size required for 4:3 is 480x360.
For 16:9, you would use FIXED_HEIGHT, so the width is variable:
Fixed height 320 / 9 = 35.556, multiplied by 16 to get the width = 569. So for 16:9, you would need a background of 569x320.
So, to make sure both 4:3 (using FIXED_WIDTH) and 16:9 (FIXED_HEIGHT) show no black borders, you would create a background that is at least 569x360 in size.
Now, there is a new problem for you to factor in to this, the iPhone X, which has a resolution of 2436x1125, so a ratio of 2436/1125 = 2.16533 (let’s say a ratio of 19.5:9).
To cater for that, you would use FIXED_HEIGHT, so to make sure it works without black bars on the sides for this ratio, you would need the following size:
320 / 9 * 19.5 = 693.33 width.
So, the final background you would need to supply to make sure you get no black bars is 694x360.
If you look at the link you put in your post, you’ll see that it’s the exact resolution they suggested too, and the bit of code I supplied in my previous post is what you need to make sure you get no black bars, as long as you use a background of 694x360.