I migrated from iCloud to FastMail about 5 years ago, and I'm very happy with FastMail. They do support YubiKeys, as well as TOTP.

Forwarding everything from the old account to the new account made it pretty easy so I only had one inbox to check, and I could take my time migrating all of my accounts to the new address. I also created a rule to sort things coming from my old account to a folder so I knew which accounts and subscriptions and whatnot I still needed to update to the new address. To this day I still have the old email forwarding, and occasionally something real comes through on it.

Fastmail also makes it easy to add other addresses on custom domains, so I have several email addresses that all deliver to the same inbox, with rules to sort them into the right folders.

It's probably because it's a business account and Google and LinkedIn have sold my address, but fwiw I receive about 100x more spam in my work gmail than I do in my personal fastmail account.