Why not retain it? It won't cost anything, and you might find a use for it eventually (I use my GMail account for automated testing of my email server).

And if you don't trust Google to safeguard your data, delete just the data.

There is probably a way migrate or at least export your contacts and email history so that you save any important conversations or contacts for future reference.

In my experience, most places have been understanding and updated my contact when I said I was registered with an old email address that I no longer have access to. If it’s a government service they’ll have other information of yours that you can verify yourself with. Make a note of deleted account names in case they are needed for verification.

If you’re really uncertain, you could set up forwarding to your new email address and retain the old one for some time without using it before you delete.

Check your password manager. What other accounts is the e-mail address account linked to? Change it, then delete the account.

If you don't have a password manager: Now is the time to get one. I recommend KeepassXC (PC, cross-platform) / KeepassDX (Android). Combine with Syncthing for cloudless cross-device sync of the password database file.

Also, don't worry too much. You wouldn't be the first to lose access to an e-mail address. Governments or "other important accounts" usually have procedures to deal with that issue. But: a Google account shouldn't be linked to anything important anyway, let alone government accounts.

Yes