Massive wall of text.
So I’ll respond based on your title.
The vast majority of player power is coming from skill setup and paragon boards.
Your gear gives you modifiers and multipliers, but rares are so important because of the +skill they provide, the % modifiers and item power.
The aspects/legendary affixes help provide your build with synergy. Uniques (1 at a time) define your build, and paragon empowers it.
At level 25 all you have to work with is less than half your skill points and legendaries. That’s why legendaries feel so powerful.