How about waiting for the G5 with CO2 (non dispersion IR type)? The cheapest one out there is 350RMB, not cheap. Seriosuly, just open a small window gap of 5mm for your bedroom window, CO2 levels will be ok liao, don't waste the money. That'd set up/increase the airflow between the window gap + MBR toilet door/bedroom door gap.
I have measured that an aircon-ed BR set at 26 deg C with 100% closed windows (mine is with excellent sealing, in fact it's double gasket) relying just on the gap under the BR door would raise and maintain the CO2 > 2000ppm in just 2-3 hours. In my MBR, I measured peak levels of 2800ppm.
It was previously thought/discussed that this is sufficient to keep the CO2 levels, with the same thinking that external PM2.5/haze can find its way indoors even with 100% closed indows, hence CO2 levels would not be high.
Is there a "feel-able" difference between 2500ppm and 1000ppm? My answer is yes, though it can also be argued that it could be a placeabo effect. There is a slightly better ease in breathing, less stale air if you will, at 1000ppm compared to 2000+. So if you want an answer, this would be it....
YES 
PS. The window gap of X mm really depends on your home. My unit is high floor and unblocked, there is decent wind most of the time as the units' windows/yard doors are facing both NW-SSE and S-ENE general directions so the "catchment" captures both monsoons aided by the block orientation. If yours is landed property, you may need a bigger gap @ perhaps 2-4cm?
