500 GB unaccounted for and Apple Storage (in Settings) never finishes "loading items"

(I'm running Sequoia 15.2 on an M1 2020 Macbook Air.) See screenshot below. It's been like this for two days. Part of the problem here is presumably that there is not enough available disk space to complete the analysis (only 40 GB free). But that's precisely the reason I am trying to look in Storage to find out what's taking up all my space! In contrast, OmniDiskSweeper is only able to find 1.4 TB (see second screenshot below), so there is about half a TB unaccounted for. That's a lot of missing space! Any ideas what's going on or what I can do about it?