If you target the boss and use Shift+F it'll be desegnated as a focus target, more or less a secondary target on your screen somewhere (can be moved via the HUD settings menu) - you can see the name/hp/debuffs/cast bar with that.
The only downsides are if the boss teleports (all the primals do) then you will lose your target along with bosses that have long names partially obscuring the cast bar even if you upsize the focus window.