Yubico's U2F security key (good for FIDO2, WebAuthn, etc.) is $25, each member of your organization needs only 1 key (if they lose their key, they can get another one from IT, which can remove the old key and enroll the new one for them), with a handful of IT personnel possibly having more than 1 key for backup (this is less necessary when a group of IT holds admin permissions, as they serve as key backups for each other). $25/key amortizes out to well under $1/month considering that keys will last for years and can be transferred from one employee to the next when an employee leaves the company, and is of course usable for any vendor that supports hardware keys.

Much, much cheaper than $21/user/month for GitHub Enterprise. I'm not sure what universe you live in where buying hardware keys is expensive compared to Enterprise licensing?