"more time" are the keywords there. Balancing this kind of game is quite the challenge as I learned during the course of making it (too many zombies = too much money?). I went through several iterations of tuning it and where it landed is probably the best it was throughout development - challenging at first, but not hard to master with a go or two. I think the speed at which the fastforward is set should probably be the default speed, and then a speed-up would actually be even faster than this. I think some visual indicator on how many zombies were spawning in a wave would help it feel less like a "when will this end??" when you get to the last few waves.
Theres a big backlog of stuff I couldnt get to and I've come to realise thats risky for this type of game in a jam because there is always going to be something 'more' that you could do, but I prioritised with the time I had available. The good thing is that as a basic template I now know how to make tower defence games lol. Im thinking of making a Christmas inspired one next and provided I can find the assets should be able to do that pretty quickly and produce a more polished result than this.
Thanks for playing and hope you had fun. Oh, and good catch on the Thriller vibe - that was the brief to the musician who did an absolutely incredible job of creating the vibe. The game really started to feel like a game as soon as the music was imported!
Edit: I also purposefully made it so you could do other things while you kill zombies xD go watch YouTube or whatever and the game will keep going. So my genre mashup includes "idle clicker" hehe.