The solution to this - I’ve since confirmed by recreating the issue and fixing again - was:
Disabling the WP-RocketLazyLoad feature.
I’m not sure how this conflicts (not my area of expertise), which is why I was hesitant about posting as the solution, but after testing this repeatedly, I can confirm blog images will fail to load on my setup when WP-Rocket’s LazyLoad feature is enabled.
I tried FlyingPress previously and had the same issue (worse actually, because ALL images forums+blog stopped loading) so I switched to WP-Rocket.
Now WP-Rocket works with Lazyload disabled. (Lazy loading is built-in by default a lot now anyways)
Current settings which don’t affect wp-discourse images in the forums: