Great news...I have found a solution. The bad news is that I think it’s unnecessary. I said earlier that I am a very heavy user of reminders. I have about 35 lists between two different iCloud accounts (my personal account and one shared with my wife) and in total have about 25,000 completed tasks (reminders) that we’re in those various lists.
My “main“ list is just called “Reminders” and I believe that’s the default list that Apple set up so many years ago. Long story short, that particular list had about 18,000 completed tasks/reminders in it. On a hunch, I took all of my active tasks (yet to be completed) and moved them to a separate list and then deleted that entire list. It took several minutes to do that. I then set up a new list also called reminders and went into settings —> reminders on iOS and changed my default list back to the new “Reminders” list that I just made.
I made this change a few hours ago and since then I’ve made about 10 time based reminders through Siri and have not had a single failure.
My Plan going forward is to take the rest of my more heavily used lists and delete them and then reset them up as a new list.
hopefully this helps everyone else who has this problem. (Essentially you are deleting all of the completed tasks since there’s no way to do that other than individually removing them one by one which is not really sustainable with 18,000-25,000 completed tasks.)
Apple - if you’re listening, please reach out to me and tell me if there’s any way that I can accomplish the same thing without deleting these lists and setting them up as new again. Again I have about 35 different lists and it’s going to take a while to move items from one list to another one, delete the old list, set up a new list, and then move those reminders back. Thank you!