Random letter groups are meaningless and boring.
I suspect the various Koch-method trainers would all be more effective if, instead of spewing random characters, they produced random words, abbreviations and callsigns (using the subset of characters they're trying to teach at any particular stage) instead...
I suppose that eventually I will agree with AC2EU after I learn all the letters, numbers, and punctuation; but not right now.
I agree with M0LEP and can pass along this comment.
"JustLearnMorseCode" by LB3KB has this feature.
From the main-menu, use Source...Entered text; you can enter in whatever words, call-signs, etc you want.
BTW, I have no connection with this code other than being a happy user of one week.
73 Jerry KM3K