Interestingly I read that whole relationship very differently.
To me, Steve's status as a soldier and a fighter who participates in the war - but also someone who is capable of great love, compassion and self-sacrifice - was important to Diana's ultimate understanding of humanity, but it didn't dictate her actions. As she tells Steve earlier in the movie, "What I do is not up to you." Fighting and defeating Ares has always been her goal and she continues to pursue that.
But what Steve's sacrifice ends up doing is providing a powerful counter-argument to Ares' attempts to convince her that humans deserve to be wiped out because they all have the capacity to wage war and inflict great horrors on each other. WW and Ares are gods who fight over the fate of humanity, and Steve is the human that makes the case for WW's side of the argument. It gives her confidence that she's doing the right thing, but it doesn't remove her agency.
Their relationship is deeper than a simple romantic love story, I think - it's as much about Diana's gradual awakening to humanity's inherent balance of good and bad (and the potential to do both). It's love on a wider scale: the compassion of humanity as a whole, rather than simply the love of a man and woman for each other. And I think Steve's expression of love for her is as much an endorsement of her overall approach to the war as it is a personal expression of love.