Using the site logo block as favicon is not a great workaround. If you have a different favicon than your site logo (different shape, simpler, etc) then this produces an issue as the favicon design is then reproduced anywhere a site logo block is used, meaning you would have to instead go in and manually place a different image file in all templates that use that block. And testing out the method above, I found I cannot then delete the template to back out. Had to delete from the wp_posts database table to get rid of it. I hope that WordPress developers will take into account that a site might also want to use different icons served for different devices, e.g. apple icon might be different from app icon and I hope the next iteration has an area for advanced customisation needs like this. It seems I will have to make a child theme and manually modify and hardcode the header, which is not great for my non-technical clients.