There is also JSON: https://jsonfeed.org/
When I wrote my own static site generator [0] I added JSON first because it's dead simple to do so. Later I added RSS and it was quite some work to get it right ([1][2]), as in I wanted both the Perl and the Python version to output an identical feed. Last time(s) I checked, they do validate for my tumblelog [3]
[0] https://github.com/john-bokma/tumblelog
[1] http://johnbokma.com/blog/2019/10/09/hand-coding-an-rss-2-0-...
[2] http://johnbokma.com/blog/2019/10/09/hand-coding-an-rss-2-0-...