Couldn't you just write that as r3k1b1 (b=bonus dice; or you could use e=extra dice)? If you use m as minus and p as plus modifiers, then you could have, say, r3k1p1b1 (roll 3, keep 1, +1, bonus 1).I actually considered changing the names, and this is probably the best suggestion in that line, but I hate changing things that have a known name unless I have to. It's on the table.
I can't use rXkY because at the risk of making the game sound complicated, there will be times when you are rolling more dice than normal in addition to potential advantage/disadvantage. For example if you have a stat at 3d+1 and two advantage, but a buff that makes your stat have +1d. Now you'd have to convert to 6dk4. Basically the number of dice you are keeping can change. I need to represent the advantage amount.
Generally no. In my text I write the dice as 1d, 2d, 3d, etc. I was only putting the "6" there to avoid confusion here on the forum.
Although it may make sense to put the modifier last, so it's actually r3k1b1p1 (in which case, you could easily render it r3k1b1+1, etc, if the letters are getting too much).
Though it is getting a bit complex, so shortening things might not make the most sense at this stage anyway. Clarity might need to trump brevity?