Ok, so here is the 'example of skill' you asked for:
1) Maybe not every top player would agree with me, but I'd always keep 11+10 rather than 11+8 because with the latter there is no way to have a good 43 or 44 long route in 10 or less turns without having a big amount of locos and/or super weird colours (9 green and 9 red or 9 red and last turn trick which means you have to be the one closing, so harder when going second, etc).
2) Sorry but no, 11+8 is way worse than 12+13. This changes if we choose the better option (11+10) because they have pretty much the same speed potential and 12+13 is more points, but I think 11+10 gets less interferences in average. Anyways it's debatable, honestly I'm not even sure what would be better.
3) It's way easier to win with my set rather than the one you originally mentioned (21+16). They have similar speed with their optimal route, but 21+16 is way easier to fail due to blocks, and when you fail even just one ticket, it's probably over. The other is more flexible and when contested loses less points. You could even play the 11 alone, close in 9 turns droping the 10 and win sometimes. With 21+16 instead you are forced in a pretty narrow range of options and as soon something goes wrong (and believe me, it will pretty often if you play against a strong opponent) you are way slower, if not dead.
For reference, this is the game the screenshots are from: https://boardgamearena.com/table?table=915663564