Although its best to keep an eye on CO2 levels using a drop checker, your fauna's behavior should be a reliable indicator. Lethargy is a sign that CO2 levels are high but may not be critically so. Fish gasping for air at the surface is a good indication that your CO2 levels are critically high and permanent damage and/or death is imminent.
With the plants involved in your expected CO2 cutoff, I highly doubt that you will experience any sort of a die-off. Most of these plants flourish in low-tech setups. Maintaining CO2 levels has more to do with keeping an equilibrium to maintain growth in demanding plants whilst keeping algae in check.
Thus, cutting back on your dosing (probably likely for the same reason(s) that force you to anticipate the CO2 cuttoff) and lighting will help avoid an algae invasion.
Fortunately, the fact that you have DIY and the plants that you have, your tank has probably already demonstrated stability despite the fluctuating CO2 levels. This would not cut it in a tank with more demanding plants that require a stronger fert regime.
You should be fine