Two comments:
#1 - I think you should consider scaling up the resolution that the player selects, instead of just scaling up the background. It looks not good in modern monitors. Example at 2560x1440: https://i.imgur.com/Yjt8BVz.png It is doubly-jarring for anyone on ultra-wide, but as they are not a highly-represented population in monitor studies, not a big deal. (I use 3440x1440, but I am part of a mere 2.85% so am not relevant.) The August 2025 Steam Hardware survey has 1920x1080 monitor resolutions making up 54.4% of players, and 2560x1440 making up another 20.2%. Here is the relevant section: https://i.imgur.com/wkxDJop.png
#2 - Upon exiting the test run to show what it looks like at 2560x1440, it did not put my monitor back to its normal resolution. It instead popped an error message that it did not support that resolution and left me to go into my monitor settings myself. That part is very, very much not OK for players to have to do every time they run a game. The error told me to contact the developer, so here I am! :D