So in order to get the padding control I need to have add_theme_support( 'custom-spacing' ); in my functions.php file. But to get the margin controls (which share the same panel however) I need to add support for it in the theme.json file, which 99.99% of all themes available don't have it, and don't use it? Somebody, please correctly me if I'm wrong...

BTW, padding/margin controls should had been handled 3 years ago, not now, where almost all block libraries have implemented their own versions of them, making the backsite more confusing and complex to end users (content makers - web designers, etc...).

Also, there are only 16 (sixteen - XVI) FSE themes on wp.org (so they do use theme.json I assume) but most are just embarrassingly horrible, as all the WP default themes with the only probably exception of Twenty Twelve - (at the time 9 years ago). I can't use any of them for a customer site (at least with a straight face)...

I am going to stop here so I won't break any of the rules here... but seriously though, as a theme/plugin developer (not a user), I'm 40% excited and absolutely love these new things, and 60% frustrated, angered, which sometimes melts to pure raw "postal" rage, and then back to being excited - it's a true love/hate story ! BTW, my frustrations are only 60% thanks to ACF Pro, where I can build my custom blocks exceptionally fast, and exceptionally easily.