Even keeping the bedroom door open helps. I did some tests with a CO2 meter - with the door closed it reached 1100ppm after a nights sleep. With the door cracked open it was around 700ppm.

One thing is that air diffuses out fairly effectively even with the door closed, presumably through gaps in the door frame. Quick calculation (welcome any corrections, numbers pulled from quick online searches):

Resting breathing rate is ~6 liters/minute.

Sleep ~8 hours = ~500 minutes = 3000 liters exhaled.

CO2 exhaled is 5% concentration, so 150 liters of CO2

Room = 30 m^3 = 30000 liters

Expected additional CO2 concentration in morning in perfectly sealed room = 150 / 30000 = 0.5% = 5000ppm

Actual additional CO2 = 700ppm (1100 reading - baseline 400ppm)

A few cm are enough

I agree, I've measured this as well. Window in adjacent room (with open door between rooms), window opened 5-10cm, CO2 level stays pretty much down.

I've also found that when all windows are closed, an air purifier seems to help (or a fan, lowest level is fine). Obviously it doesn't remove the CO2, but it seems to prevent buildup in the one corner where you are (bed, desk). I'm sceptical it's a problem but the data shows it is.

Yes, modern buildings are really well sealed compared to before. A few cm is an enormous gap, relatively speaking.