@schwa Do you still have this problem?
I ran into it after restoring a full backup from Time Machine to a new Mac; Sequoia on both source and target. Reminders was entirely blank on my new Mac (no entries in the sidebar, like in your case).
This helped me (the post that I link to, and the one that it quotes): https://forums.macrumors.com/threads/reminders-app-on-mac-blank-after-migrating-to-new-mac.2393387/post-33856362
The main difference was that I killed “remindd” in Activity Monitor *before* deleting the directories, to be sure that it didn’t try a sync before I rebooted.

I had this same problem and can confirm that kirbyrun's solution worked for me, i.e. emptying the /Users/USER NAME/Library/Reminders folder was not the answer. In the Console app there were error messages generated when opening the Reminders app. For example: error: (11) Fatal error. The...
@schwa That’ll do it! 😜 I don’t know my way around all of the various container directories, so my nuking wasn’t successful until I’d done it in the places that those people listed in their posts.
I pine for the days when holding down the Option key revealed in-app reset functions. It was still a non-obvious thing to do, but it at least avoided having to wade through a potential minefield of preference and state files.
I’m glad that you were able to work it out!