Agree with @Sui-hin Mak, start is a tad tedious while mid-late game you blaze through upgrades. Enemies providing no real challenge. Game Jame wise, it's a solid game.
Honestly, the only issue is the difficulty curve; next time what you can do, is give yourself all upgrades and set the stage to 10. Now adjust ghost health just enough so you barely finish stage 10, this is your end-game difficulty and final stage. Repeat same logic for stage 0(No upgrades) and stage 5(50% upgrades)... Add a non-winnable stage 15 if you want player to be able to finish an extra stage or two. If you want something more endless, after Stage 10. Increase ghost health by 2-10%? per extra stage after 10. I'd suggest ending the game at wave 10-12, endless only works if there is some infinite player upgrade or my favourite "last stand".
Create a 2D Curve in your given program with the values you calculated.
X-Axis(Time) = Stage, Y(Value) = GhostHealth
For example: stage 0 = 1HP stage 5 = 2.5HP stage 10 = 12.5HP stage 15 = 37.5HP
Now you can query the Curve Time(stage) to get the ideal GhostHealth or "Difficulty". Adjust curve handles and add more points as needed -- just like with animation, the more points(keyframes) you have the more time consuming and more restrictive adjustments become. Try to keep as few points as possible.
Ramps up slowly in the beginning and super fast mid-late game, due to the nature of incremental games and having lights hit every ghost in contact. They will need a lot of health to get anywhere close.
Mid-Late upgrade costs most likely will have to be increased(Can use the same curve as above, just make sure to normalise it). Or reduce bonus soul value; player already gets more souls as Ghost spawn count is increased.
Extremely hard to get player engaged just👌, when upgrades tend to be exponential. Going from praying a gust of wind doesn't blow a white sheet in between your lights to nuking shadows out of existence with disco lights.
Found your game via Itch New & Popular; good luck with the Jam. Had fun playing it.