While a plasma is far from an ideal gas:

pV=nRT

p is the pressure, T the temperature, when you increase the pressure while keeping everything else the same, you increase the temperature aswell. The density here is the colloquial term for pressure.

I dont actually know because I make (non-Whatsapp) calls like once a month or less, so whatever I do my bill stays below 20CHF/month. But yes agreed, sounds like a cheap plan for a power user like you!

40 CHF/month is a super expensive plan, I pay <20CHF with spusu, unlimited in CH and 10g, 100 minutes, 100 sms in EU

Root the phone and set up something like a wake call/minimal cron job every few hours? This should keep it from hibernating and thus pausing the process for the notifications.

I would recommend anything Thermal Grizzly: https://www.thermal-grizzly.com/

Simply because the owner has two 1 hour long videos explaining how its made and tested and it actually seems to be one of the best.