I have been playing with Shortcuts to limit the charge level of my iPad to around 80%. I am not sure if it makes a difference to battery service life but I thought I would get some practice with Shortcuts.
The trick is to create a Personal Shortcut and have it detect when the iPad charge rises above a certain level then turn off the Homekit power outlet that the iPad charger is using.
I have created a sequence of screenshots to show the key steps:
Note I haven’t tested this yet as my iPad doesn’t need charging…