I want to stop using Mail (from iCloud) on my Mac. I only use it on my iPad. If I ever need it on Mac, I'll check icloud.com.
So in Preferences section Apple ID, I've deselected iCloud Mail. And in Preferences section Internet Accounts, I've deselected Mail for each and every account listed there.
But when I check Storage Management.app, I'm still seeing 5+ GB of Mail. How do I delete that?
Unchecking the box of mail Set up iCloud on all your devices - Apple Support will stop syncing and mails will not be visible , but when you check iCloud storage Check your iCloud storage on iCloud.com - Apple Support
Follow these articles Free up storage space for email accounts in Mail on Mac - Apple Support
Keep mailboxes from getting too big in Mail on Mac - Apple Support
Unchecking the box of mail Set up iCloud on all your devices - Apple Support will stop syncing and mails will not be visible , but when you check iCloud storage Check your iCloud storage on iCloud.com - Apple Support
Follow these articles Free up storage space for email accounts in Mail on Mac - Apple Support
Keep mailboxes from getting too big in Mail on Mac - Apple Support
Remove Mail from macOS to clear up space
