It's entirely possible that my best fix is just to delete my haveibeenpwned account and react when I get spam, but where's the fun in that?

The founder was asked to provide a subscription level for individual domains and he said no and pointed people at the suggestion to search manually or occasionally pay for a month instead.

HIBP subscriptions can be taken out monthly and cancelled at any time. If the appearance of your domain in a breach is infrequent, you can take out a one month subscription then immediately cancel it after performing the search (the subscription will remain active until the entire month period has elapsed).