You won’t get the option of half Haste, half Mastery. You’ll take the gear thst is available.
I would say: at each piece, choose the gear that is best for the spec you are playing most at the time. When you have highest-ilevel pieces in all slots, go back if you want and pick up the pieces that would be a significant upgrade for other specs, then use them when playing that spec.
Ar higher level play, it is normal to have a different set for each spec.