Funny. I've been 1h cav / shielder for 12 gens now and although i've tried 21/15, 24/12 and simmilar builds, my conclusion has been that 6 athletics is the sweet spot for a versatile shielder.
So i'm 18/18. Slightly unforgiving when taking hits, yes, but generally it allows you to do well against multiple opponents, chase archers and stuff like that.
It's highly personal though and clearly depends on what style of play you prefer. I can imagine in tight group play that the extra athletics isn't needed.