The amount of background crap has been steadily increasing on Apple devices, you're probably right in your observation that it's related to wireless/Bluetooth.

I have a iPad 2 (yes, the one from 2011) which I keep around with the plan to turn it into a house-automation screen and I'm always surprised when I look at it every 2-5 months to see it still has a decent chunk of battery left. And it's in sleep, not turned off.

I just checked it, it's at 76% with a standby time of 562h (23.4 days). And this is with WiFi turned on.

Im really impressed, it's after all a 12 year old battery. And the device was in daily use for the first 4 years and then less frequently.

This is a side topic but I find it's a good example of forced obsolescence, because there is nothing wrong with it other than the fact that it does not support modern web standards or modern iOS apps which makes it trash for most people.