I see, that’s very interesting knowing that the game was initially harder. If you’re trying to balance one difficulty for everybody, it seems sensible to me to err on the side of making the game easier. Mainly because you don’t know what type of players you’ll have, but probably many will be tower defense novices (such as myself). From my perspective I’d rather play a game a bit easy than too hard, which becomes frustrating. Especially for jam games where you want to have time to try as many games as you can.
That option of novice/expert you mention would already go a long way in making balancing easier, as well as potentially extending the play time of the game. I haven’t tried to implement those systems myself yet, but I’d imagine they’re not trivial to implement so in only 9 days it’d be hard to find a lot of time to invest in that!