you could make a custom chapterbackgrounds.txt file and put it in /tf/custom/yourstuff/scripts/ and have different backgrounds for each "chapter" (background)
backgrounds go in /tf/custom/yourstuff/materials/console/ in the vtf format
Looks something like this:
https://i.imgur.com/vrChLZk.png
instead of background_upward for each entry you can change it to match the name of the vtf in the materials/console folder. i.e. background_custom, etc.