Don't think you should enable detrimental gameplay (currently, you want the rate you consume Ash to be able to keep up with the rate you produce it, you wouldn't so much want it lagging behind just to be able to get that buff- it would kind of mean you *never* want to upgrade production rate and just bank millions of Ash
Can't say I particularly use steam much these days but if the keys don't expire I wouldn't object to having one around
There's obviously the options of Weapons Development/Research but not sure it would be what i'd recommend- things like chemically extracting grease from blood instead of just squishing, or fuelling furnaces to process Iron and get another multiplier to the rate - it really really really depends on what your vision for the game is, let alone scope or true end goal - Restoring the world without people, just plainly destroying the planet etc
Hell, induced earthquakes even, who knows (if it was some global effect maybe the surviving pops could develop resistances to it over time)- what is the society you have in mind with this etc, where we can capture and forcibly execute people - much of it wouldn't even need to be included or mentioned, just an overarching picture in your head