Why are my Calendar alerts not working on iPhone?

I have an Iphone 13 Pro Max that I recently updated to ios 18.4. In the past 3 months or so, it has stopped giving me calendar alerts for things that have alerted me for years. I get no screen notifications, nor do I get sounds. If I add an alert to my calendar, it will alert, but some prior alert will stop working. If I update an alert that is no longer working, it will alert, but something else will stop alerting. It feels like I have hit some kind of limit with my alerts. (I have quite a fair number of them.) Some alerts are set weekly, some monthly, some daily, some every 3 months or so.

I have googled this issue many times and have tried all of the recommendations, but nothing fixes it. I am beyond frustrated and ready to get rid of my phone and switch to an Android device.

[Re-Titled by Moderator]