I find that using the keyboard shortcuts in the Identify modal to annotate observations is plenty fast, especially if you filter observations correctly (eg by taxon). I made a tutorial for this here: https://forum.inaturalist.org/t/using-identify-to-annotate-observations/1417 In fact, it’s so fast that I will sometimes accidentally be on a roll of marking lepidoptera as adults and accidentally mark a larva as an adult.
I do agree this would be helpful, and I made an request for it to be added: https://github.com/inaturalist/inaturalist/issues/2353