..and it's not clear to me that it's 'fact' that CO2 correlates so closely to algae.
Yes it's way to general. This is 'The Planted tank' not 'The Heavily
Planted Tank. The CO2 will only buy you so much since it's directly related to plant mass. Of course light, waste load and other factors are at play, but I see so much advice to correct CO2 before anyone even sees the OP's tank. Why does CO2 work at all. The plants increase uptake and get rid of nh3, etc in the WC. It's not an Algaecide like the way Excel is used. It needs good plant mass to be effective.