I haven't opened this game up in the engine in a year so I could be wrong on the specifics but you are mostly right!
Crop dust should add an extra bullet with every level, they are heavily stacked though.
I think Clap does more damage based on level, which is only really useful with the stronger wisps.
Flickering rate increases, honestly just making it hard to see during late game.
+Wisps increases the amount of wisps that spawn whenever you run over a haunting point.
+Absorption increases the amount of XP you get from the XP pickups. Using it along side a high +Wisp level in the parking lot is the fastest way to level.
Difficulty increases in two ways,
1st are the clocks. Once you hear a clock chiming (by running near a clock for the first time) it will increase the chance of wisps evolving from their 1 hp variant to their 5 hp variant.
2nd are the haunting points, usually doors will change the wisp warning in the top right to spawn a certain number of wisps (+Wisp level).