I use LaunchBar on the Mac which allows one to do many such things without leaving the context of work. For example, I would be commenting here and some idea strikes me suddenly, I quickly use my shortcut to bring up LaunchBar (which looks like Spotlight/Alfred/Obsidian command palette), enter text mode, type something quick, hit tab, pick a file, select append to file and hit enter. It seems like a few keystrokes, but it’s the quickest path to typing something to be appended/prepended to a file.
Your request/idea gave me a thought of how it might be implemented because I see some issues with your method of using :: to add content inline. Firstly, if the note already exists, how does Obsidian handle this text? Always append to the end of the note or have a preference to select prepend/append/replace? Secondly, since typing the [[ puts the cursor before the ending ]], once the user types the note and content and moves the cursor beyond the last link brackets, suddenly removing the text might be jarring especially if it were longer than 2-3 words and also how does the user know that the text was successfully added to this note?
So I propose a slight modification to how it might work while maintaining your original incredible idea. The user after having typed the note title [[term x]] could press the hotkey to bring up the command palette which has new actions like Append text and Prepend text. Selecting them and hitting tab allows to enter text inside the command palette itself and hitting enter would commit where the command palette can show help text notifying that the text is being appended/prepended to term x note. The actions themselves can have assignable hotkeys so the first step is eliminated entirely.