Even my iPad has horrible background drain. My iPhone is the only device I have that I can calmly leave at 15% when I go to bed and know my alarm will still work in the morning.
I "upgraded" to a Pro 2 years back and the first thing I noticed was it'd go from a 100% charge to 60% to even 40% overnight. All apps closed, freshly rebooted and did nothing but charge it the previous night before unplugging it.
A lot of comments online say "This doesn't happen. You're doing something. And it's certainly not wifi and bluetooth because those don't use battery while idle so do NOT turn those off!" I tried everything that was recommended, turning off iCloud, uninstalling random apps, etc.
Then I got into a habit of turning bluetooth off every night and my battery would only drain about 4-6% a day while idle. Turning off wifi as well and I get about 2-3% drain per night. I have no clue what Apple is doing, but when putting my iPad to sleep at night with any sort of wireless on, the device connects to every fucking thing it can find but wouldn't do while awake. I never even synced my Airpods with my iPad and made sure to delete them when they did connect, but still, if I leave bluetooth on before going to bed, my iPad will connect to my airpods while they're in the case, drain them to 0% (even after fully charging the Airpods before sleeping. It's not even possible to drain the case's battery with actual usage that fast!), then drain 40% of its own battery.
I can't make sense of it.
In short, turn all wireless off when you're done for the day. iPads/iPhones are annoying in that you can't turn them off from the drop down menu though, so you need to go through settings or make a shortcut.
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.
It's still fine to work on though. So have no need to replace it, but it is a little annoying to find it at much less of a charge than what I would expect.