I usually edit posts for grammatical and spelling errors and strongly believe that most edits are for the same reason. I do hate typing "fixed grammar", or "improved formatting". So, why wouldn't Stack Exchange add a drop down list for this purpose. The list can contain the common purpose of edits, and miscellaneous issues can be typed as usual.
I would suggest to use your browser's autocomplete feature. I use it on Firefox, macOS, and I know it works for Safari too:
For some browsers, it seems to work out of the box, in other ones you might need to enable it explicitly.
Also, once you reach 2k reputation you don't need to type an edit summary anymore; the system will automatically generate one for you. Edit summaries can be very useful but grammar corrections and fixed typos are usually quite obvious.