Guess it's not just me:
@schwa It feels like anyone who knew how to do UI retired, and the new team said, let's rewrite the whole thing and shoot for maximum backwards compatibility "as speced" so we can clear that 25 year UI backlog at the same time.
I'm not a dev manager, but I've been managed, and this is what it feels like
@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!