Assuming I have no idea about these things, which is true—I don’t—how much is a decent CO2 sensor that can be connected to a Pi or similar setup?

I recently upgraded to double glazed windows in half the house, including all the bedrooms, and put a heat pump in each of the two used bedrooms. So I want to keep the windows closed for heat retention, but also want to keep them slightly open to reduce CO2 build up.

I sleep in my small bedroom with my two 16kg dogs, so it seems reason the CO2 level may rise to suboptimal during the night if the window is shut.

What I would like to do is make a DIY ventilation system that can move fresh, HEPA filtered, air from outside, in to the room.

Or am I over complicating things? Could I just use a small fan, something sized to replace, say, half the volume of the room every few hours, and just leave it constantly on? Or should I put a larger fan on a timer so it runs for a few minutes every hour?

Does any of this really matter?

Edit to add: any good resources to lose myself down a rabbit hole?