For your consideration, one does not need to have their password manager online to use HIBP; they offer [at least] two different concessions to your concerns:
Thus you could hash your passwords in your airgapped setup, transfer the hashes using a mechanism you trust to an Internet connected device, and then check the hashes
Just use a unique complex root password for your password manager and check semi-regularly that it hasn't leaked on haveibeenpwnd.
Bonus points if your password manager automatically checks your stored passwords for leaks and scores them (eg. LastPass)