Terrible destrier build. 18/18 for destrier? No, no no. Go 21/15. The 1 more riding skill isn't worth it.
If you want 1h/shield and polearms, go 21/15, if you want only polearms, go 24/15 (at level 30).
You can thank me now.
Also, I never understood why the "default" crosswbow/2h build is 15/24, why wouldn't it be 24/15?