This does seem very epic! I agree it seems you need a bit more reward especially if you had to cheat some of the enemies away in testing.
I'd love to have other things that have this sort of mechanic and really extend this idea further, whereby using said items caused a wave of zombies to spawn, getting harder and harder the longer it is left running.
A great example would be a POI where you could find a big blast furnace that is capable of turning forged iron and coal directly into forged steel at a super fast rate after 3 waves. You deposit up to 500 iron and 500 coal in there and get 500 steel out at the end. However by using this item it would cause waves of zombies to spawn around the POI with the blast furnace inside, and the longer it is left on, the more zombies will begin to spawn until you even start getting rad cops and eventually bosses, if left on for way too long. This could also be the only place you could craft a crucible, enabling you to make steel ingots in your own big forge. For that you'd need to survive 5 waves of attacks.
And what about doing this with some of the nuclear plant systems? Have it so that when the turbine turns on, it may have a chance to summon zombies around the plant that would try and break in. Would make reinforcing the power plant with traps and camping there necessary if you wanted to use it for power cores to power the exo suit later on.
You could even use this sort of thing as a gate, whereby a player would have to venture out to find the POI that had an item capable of making the next thing to progress. Then have a progression something like
Early game -> Steel POI -> Mid game -> Nuclear Plant POI -> Late game -> Rad zone
I remember you guys saying the minibike was OP as well, what about having the minibike chassis made on an 'improved lathe' which you can find inside a working stiffs store or pass+gas randomly? Another way to have this mechanic be used. You want that minibike? You're going to have to fight tooth and nail to get it. This would also force people to walk through biomes, encountering animals that the bike lets them drift right past.