How do I delete thousands of emails from my iPad?

Often the easiest method by which to bulk-delete email is to use a web-browser to sign-in to your email provider’s web portal - and delete mail directly from the mail server.

Otherwise, see >>> Delete emails on your iPhone, iPad, and iPod touch - Apple Support

Tap the Edit button at top-right of the Mail sidebar; simply swipe your finger down (or up) the left-side of the message list to bulk select Mail Messages. Swipe below the bottom edge to rapidly select all the visible email messages.

Once selected, tap the Trash/Bin button at the bottom of the sidebar.