Note that the behavior of serve_from_sub_path = true in grafana.ini seems to have changed in 10.x.
I’d previously had a working setup with 9.something, Apache 2.4.x, a ProxyPass reverse proxy setup, and serve_from_sub_path = true. After upgrading to 10.x, I needed to comment this out (reverting it to its default of false) in order to stop the endless redirect loop. Nothing else about the Apache configuration was changed, or needed to be changed.
I don’t claim to understand what this setting does, except that #70110 on GitHub mentions it (specifically this comment), and altering my config back to the default allowed me to reach our Grafana site again.