Standby time is a plague of iPadOS that is only getting worse in my experience. I have 2 pros on 18.5, 1 on 17.7 and one on 26 beta 3 (as well as 2 mini 5 on iOS 17 and 16) and while the beta is atrocious, we'll give it the benefit of the beta, but the others are pretty bad too... We are at a point were my Android tablets (tab S4, S7, S8 ultra and S9) have all way better standby time than iPads (1-2% overnight vs 5-10%).
And even Windows tablet do better but they cheat because after some standby time they hybernate unless you disable that).
Ugh, yes. Standby is just awful on both iPadOS and iOS on my devices even when brand new with no firmware updates. The only thing that reduces it is airplane mode (cellular, wifi and bluetooth all off). Even on low power mode, standby usage is still high (culprits being Find My, Home and Siri).
I've taken to scheduling a daily charge at 5am using smart plugs (with shortcuts to stop charging at 90-95%) to make sure my devices have sufficient charge for the day.