Worrying about battery health on iPads is a waste of time, in my opinion. Hell, worrying about battery health in general is a waste of time. On iPhones, if you don’t update iOS, severely degraded batteries are just fine (my iPhone 6s on iOS 10 with 63% health and original battery has almost like-new battery life); if you update iOS, battery replacements improve the situation but are largely irrelevant anyway. Let’s take the same iPhone, the 6s. Degraded batteries on iOS 15 are abhorrent, yes, with many reports indicating about one hour of screen-on time at best. Replaced batteries... are pathetic as well, with people reporting 2-3 hours. Better? Yes. Usable as a main phone throughout the day with current standards? Absolutely not. The iPhone 6s is gone as a moderate full-day phone thanks to iOS updates. Worrying about battery health won’t ever fix that, regardless of how many batteries you throw in there.
On iPads? Even less necessary. People use 8-year-old iPads with original batteries and they retain decent battery life. iOS updates obliterate battery life, but if the user is a light user, they won’t care much. Yeah, heavy use coupled with the maximum amount of iOS updates will obliterate battery life if looked through the lens of “when it was new it was a lot better”, but people still report maybe 6-7 hours of light use years after purchase. Yeah, heavier use gets maybe 4, but for an 8-year-old updated iPad? I think that’s a lot better. An 8-year-old updated iPhone would be totally unusable, even with light use. 7 hours of light use gives the vast majority of its users a full day. If they’re enthusiasts, they probably have moved on by then. If they haven’t, it’s still decent.
My 9.7-inch iPad Pro saw an immediate decrease in battery life after Apple forced it from iOS 9 to iOS 12 back in 2019. Battery life has remained stable since then, with about 11 hours of screen-on time with light use (got 14 on iOS 9). A little under 700 cycles and 85% health. I doubt it will change significantly even if I throw another 500 cycles on it. It remains to be seen.
I’ve yet to try a massively cycled iPad, but I am extremely confident. The iPhone 6s I mentioned earlier has almost 1400 cycles and battery life is just fine. iPads with their larger batteries give me even more confidence. I reckon even 3000 cycles wouldn’t do much damage, at least if kept on a half-decent iOS version. Updates can always shatter these predictions, but still, like I said, fully updated, old iPads are just fine with light use, even if they don’t compare to the original iOS version.