Unfortunately, there is no RSS/Atom feed for issues or pull requests.
I made a little Ruby script (https://github.com/bhaak/github-feeds) that outputs static RSS files but you would expect that GitHub would have exported those by themselves.
You can also use RSS-Bridge[1], which covers GitHub Feeds, and lot more sites. My instance[0] has the GitHub bridges enabled, and I subscribe to a few issues on our Slack channels this way.
[1]: https://github.com/RSS-Bridge/rss-bridge [0]: https://rss-bridge.bb8.fun/
I like your static-file approach though. Never thought of pre-generating RSS-feeds that way.