I don't agree entirely. I was aggressively unlocking gifts basically whenever I had the haze to do so. The early ones mastered fast as hell because I was under leveled. Customizing the set of gifts for an area or boss became routine, and you can do it on the fly, which is huge. If you've mastered them, you don't need to shift blood codes so long as you have the stats for it, and the late game codes give you access to the majority of them. It still breaks down to Fighter/Caster/Scout for the ones that you can't, but those high-stat requirement gifts are highly specialized, or the right Passive gets you there (Mind Up -> Bridge to Glory, looking at you).

By the time I'm getting to a boss, there's nothing to unlock.

Upgrades I was also doing throughout, but they're not that big of a deal because you'll either get all the materials you need late game when you're swimming in haze and you can buy them in unlimited quantities, or you're really only focusing on a favored weapon or two. Blood Veils you just move up to the next one you found until NG+.

If you're after mastery, you want to be under leveled at all times. Leveling is really reserved to "Am I getting hit too hard by mobs?"

People can and do beat the bosses with pipe at level 1.