Update 5
Making some changes managed to break everything completely and I've no idea why. I went back to an earlier save and put the changes in again one at a time to try and find the issue, but it all seems to be behaving now. Which is kind of good, but also means I have don't know what I did wrong the first time. Probably not going to mess around with the CSS so it'll be functional but mostly plain looking.
I'd call the main bit of the coding ~80% done. I've just got to link the upgrades to their effects and get various elements to show up when they're needed, and get out of the way at all other times. At this point I feel like the economy balancing is going to be me picking random numbers out of a hat. I shouldn't be adding/removing any major elements any more so I can start replacing placeholders with actual images, so that's one of the next jobs.
I haven't even touched on saving, but it's fairly short so I could(?) maybe get away with not having that as an option, as I don't really know where to start for that.
Other than that, I need to come up with some sort of endgame, or even just a win condition to create an actual stopping point.