I have an idea:
Step 1. You add 1 more calculate inside the repeat to define which repeat is max num:
if(${num} = ${max_num}, ${letter}, '')
Step 2. You join the letter of max_num in the above calculate OUTSIDE the repeat, i.e. after the end repeat (in this example, I name it 'mark_letter'):
join(' ', ${mark_letter})
So you can get only 1 letter or more letters, depending on how many num are max.
@LN: you are right, jr:choice-name doesnot work with choice filter.