Apple Reminders Buggy between devices?

I've been having trouble with Apple's Reminders app since the Catalina update in September 2019. I didn't realize until after updating my iPhone XS that I should've waited for the update for my Mac; figured it would all sort itself out once that update was available and everything was up to date.

But no, it's been awful since then and getting worse. I have a Reminders list on my iPhone that doesn't show on iCloud.com or my Mac. If I create a reminder on one device, it doesn't always appear on the other devices. If I check something as completed on one device, sometimes it re-appears days or weeks later. It usually does not show as completed on the other devices which requires me to mark many things as complete twice (if they ever synced to both devices to begin with).

I'm logged into the only Apple ID I've ever had on both devices. I have unsynced from iCloud on both devices, turned them both off/on, and then synced Reminders with iCloud again. When I did that, I got an old list that had over 20,000 completed reminders on it downloading to my iPhone (I'd deleted it completely last year as it was bogging things down).

Finally today I wrote down every single reminder (I have hundreds of recurring reminders for work) from iCloud.com, where I *think* everything is showing in one place. I'm ready to completely wipe Reminders clean and start from scratch...but after that Reminder list from last year reappeared like a freaky ghost (of course, since it's 2020), I'm not convinced it's even possible.

Anyone have a suggestion?