how to manage storage on an ipad and still keep it on my apple account

I have a lot of content because I have a good 1 terabyte iphone. About once every few months I use an ipad air 3rd gen wifi only 64 gb synced to my apple account and because I use so rarely I don't want the memory full. I'd like to tell the mail app and other native apps to only keep locally 10 days of mail for example so the ipad won't constantly show full. Because if I delete something on my apple account it syncs that deletion everywhere and then its' gone. I don't use mac computers so just phone and ipad and I don't want to spend the time to save things elsewhere. I'm guessing apple wants me to buy all my devices with native memory to support all my content. Coders could easily make an occasional ipad useful by limiting local storage.