iCloud Mail stuck on downloading messages

New Mac Mini M4, Sequoia 15.2, Mail issue (only seems to affect iCloud mail - Outlook and Gmail accounts work fine):

iCloud mail account is stuck on downloading approximately 3000 messages and will not load them; also, I cannot simply quit Mail - it does not respond and I have to Force Quit the app.

iCloud Mail works fine on my iPhone updated to iOS 18.2.

Help!

[Re-Titled by Moderator]