Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Dear Darcelle,
About "I've also had situations where I have commented on a task, and another team member comments the day after, and I don't get a notification of that comment.", that should be an issue in the email configuration instead of Planner notification settings. The emails should be sent to the Inbox folder. This type of the notification can't be disabled.
Manage Planner notifications - Microsoft Support
In some situation, if the emails aren't sent successfully, you may receive the error message "Your message wasn't delivered because the recipient's email provider rejected it.".
So about the issue that you can't receive the notification after someone or you reply to the previous comments in your environment, I suggest you post a new thread in the Outlook category for further help.
About the comment notification in Planner, I'm afraid that there is no way disabling it and customizing the notification recipients at the administrator side. The possible workaround may be to let users set the group email settings at their side and this can't be controlled by the group owner.
If you want to customize the notification settings fully, I think that the alternative way may be to use a SharePoint Online list to create a task assignment dashboard and use Power Automate to create flows to send the notifications based on the defined rules automatically in your environment.
Meanwhile, as out product developers are willing to hear users' suggestions and feedback, I kindly suggest you submit your feedback on the Feedback platform to let them know your requirements for product improvements.
You may wait some time to access it based on the network environments.
After you access the link, you may see the following and select the category to submit your feedback.
Thanks for your understanding and have a nice day!
Sincerely
Cliff | Microsoft Community Moderator