So far we've done our best to keep a really readable, tidy Git commit log, which should help with understanding changes: https://github.com/whatwg/html/commits/master
If you omit the "Editorial:" or "Meta:" commits, I think it's actually at a similar level of detail as the W3C fork's changes log. (Not completely; scrolling through I do see a number of commits that wouldn't be relevant.) But the W3C fork has only managed to copy-and-paste a small subset of our changes, so indeed, the changes log for the last year of work at the WHATWG will be somewhat daunting compared to the small subset they managed to copy over.
There may be room for someone to compile a higher-level "this week/month/year in the HTML Standard" or similar; before I started working in the WHATWG, that actually used to exist: https://blog.whatwg.org/category/weekly-review (also in very amusing YouTube form: https://www.youtube.com/watch?v=1Bg5BPnmj68). So far we haven't had the bandwidth to restart that, but if you or someone else wants to contribute that sort of thing to the blog or elsewhere, I'd love to help you get started.