The way I tackled this option:
I created several (The amount can change to your liking - obviously more is going to be a larger file) backgrounds
Each background was a small repeatable background (Or what you labeled an infinite background)
After I traveled 12000 distance units it would change to background 2 -> then 18000 changed to background 3
This allows you to show scenario changes while keeping each one smallish
If you use this:
I suggest setting your infinite background as you already made as animation 1 - then adding the 2nd 3rd etc as new animations