Despite deleting all mail accounts (except the rarely-used default iCloud account) as well as restarting the iPad, I can’t seem to reclaim the space from the Mail app:
Any tips?
This might be that confusing thing where the data is marked for deletion but not actually deleted until something else needs the space. Macs are a bit more transparent about this when they label space as “purgeable” (though I don’t think it’s possible to manually purge them). The idea is that if you then re-enable a mail account, the phone won’t have to re-download all of the data.
Perhaps a better solution is to delete the Mail app, which according to Settings will delete all related data. Then after the space is truly released, reinstall Mail and add the iCloud account.
You can delete the Mail app from the screen above (tap on the right arrow and you’ll be taken to a sub-page where you can delete everything). Or delete it from the home screen.
I agree with @margaretamartin. Delete and reinstall the app. That’s my solution for any iOS app that gives me trouble.
If that doesn’t work, I’ll delete this post and deny saying anything. 
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Thank you for the responses!
I deleted Mail and then installed it again.
It is better, but not what I would expect:
With only ~15 GB currently available on the entire iPad, I would have thought that it would clear all the data.
Will monitor it to see if iPadOS allocates more space as needed.
Thanks again.
This does not seem right, if your iCloud account is truly rarely-used. I rarely use mine, and on my iPhone I have 4 email accounts (some of which are very active), and I’m using less than 1 GB of space for Mail — in total.
I would wait a day and see if it goes down. In the meantime, you could log into iCloud.com and make sure everything in the trash has been emptied. I doubt that will fix it, but it’s worth doing just in case.
If it’s actually purgeable space, I have seen people upload a really large file to iCloud to force the purge to happen, then deleting the large file. Sometimes this works, sometimes not.
If it doesn’t free up space, I’d suspect that something in Mail has become corrupted. Unfortunately, there aren’t a lot of options for us regular folk to fix it. It might be possible to see what’s going on by using a tool like iMazing on a Mac, but since it’s tied to iCloud, I will bet this will need a call to Apple Support.
It is possible that wiping your phone and reinstalling from a backup will release the storage or fix the problem, but it’s also possible that the problem will re-appear with the backup (especially if something’s been corrupted). I recall that years ago Dave Hamilton of the MacGeek Gab podcast had a problem with his son’s Contacts, and the only fix that worked was rebuilding the phone after wiping (instead of restoring from backup).
Let us know how it goes!
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iPadOS will probably release this space when it’s needed for other files or apps; as said above this is probably purgeable data that has not been purged yet. Storage usage reporting has also not been enitrely reliable with iPadOS 15 until very recently.
You might want to try downloading a couple of large games, such as those that are 1GB+ in size, to force iPadOS to do some cleaning up.
Quick update: Despite using the iPad as normal the last 1.5 weeks, Mail still shows to be using 10.9 GB.
Not a big issue now, but I may need to do a clean install of the OS to get that space back.
Thanks for all the help and input!