Thanks for the feedback!
You can already upgrade every 5 waves, but yeah I probably should have spent a bit more time on the upgrades themselves. Glad you enjoyed it!
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Sorry for the late response, but I went back and played up until Wave 24. The game gave me an upgrade every 5 waves; one on Wave 5, one on 10, one on 15, and one on 20 (I died one wave short of 25).
Yeah, I don't see any bugs there (unless there's some oddly specific method that triggers it). Thanks for reaching out, though.