Is it possible to disable gmail sync completely, without logging out? Is it possible to completely disable all Gmail synchronization, without actually logging out of the account?

The use case is a phone for a senior who does not read email on the phone.  For senior reasons the phone is set with no passcode to unlock.  As it stands now I've set the sync to the minimum possible (1 day if I recall), but there's still the ability to search for and find private email messages.  There are potential nosy persons with access to the phone.

See also Logout from Gmail account without logging out Google Play Store account and How can I set up my Google account without syncing Gmail?

And as everyone should know: anyone with access to a phone can reset the password on almost any online service account.  A few banks are smarter than that, but not all of them.
Disabling sync and removing email history is possible, but it will not stop others from simply enabling sync.

Option 1.
To disable sync for Gmail, go to settings -> accounts -> Google. And deselect the Gmail sync option.

Now, go to settings -> apps -> Gmail -> storage -> clear data. This way, when you open the Gmail app, it will not show the email history.

Option 2.
disable the Gmail app. Go to Settings -> apps -> Gmail -> disable. This way the Gmail app will simply not show on your home screen, and you will not get any notifications from the app.

But to fully disable sync without anyone having a way to enable it again, is not possible as far as I know.

Email could be access in many ways, like open the chrome browser and going to gmail.com there, it will not even ask for you password because the phone is already logged in.

In short your options are: lock the phone with a simple code that can be used even by seniors, or remove the current account and add a different one, or use the options above, leaving the phone vulnerable.