Some sort of organization of queries into categories would be nice.
Most of my queries don't need to be checked every 6 hours. Some are fine being checked weekly.
You could have a free tier with limited numbers of queries and 24-hour interval to attract new customers.
Someone else mentioned public quereis being free. If I can still get a personal notification for those, then you only spend on sending notifications and not re-running the query for many users. If my needs are served by public queries alone I may not subscribe. I'd weight the risks.
Different methods of receiving notifications are usually only needed for enterprises. I'd weight the benefits of doing that for non-enterprise customers because the cost of debugging is high (think hooking that up to AWS SQS or, worse, some proprietary webhook).