For bold – its possible to type control-B (or click the Bold button) then the words you want followed by END (on a keyboard) rather than another control-B or button click. That works and avoids having to go back afterwards to highlight separately. On a mobile, you’d have to simulate the END by moving the cursor but is probably still easier than going back afterwards.
The same ought to happen with control-I for italic, but currently it doesn’t due to somebody reusing control-I in the Flarum core as indent. Maybe that silly bug will get fixed one day, in which case you’ll be able to do the same for italic and bold italic.