Thanks for the detailed feedback! I agree that the progression curve is not steep enough in the beginning. I need to work on instilling a sense of power earlier.
Regarding attack rate being tied to heat: This is only in some upgrades - the more powerful ones, so they have an upside and a downside. The heat problem can be solved with other upgrades though... but still, there is a lot more balancing that would benefit the overall gameplay.
Thanks again for your constructive criticism! I'll incorporate this in the next version.