It's not only about redemption, it's about character progression within the show, and for Anya and Spike, willingly fighting for the world is a character progression, because they wouldn't/didn't do that when they were first introduced to the show. Their sacrifices may have redeemed them a little, or not at all and redemption may be a continuing process, not a 'one big gesture' thing, that's down to our (and their) individual feelings on the subject.
Yes, you're right it is about personal character progression. Redemption may be a continuing process within their character development and then, as you say, their/our individual feelings about it are key to how we perceive that journey and their progress on it.
Both Spike and Angel are seen to connect themselves to their soulless actions, and as they both retain the demon within their 'mix' alongside the soul and still feel those drives as well, I find that logical. Even though they are distinctly different when souled and neither would make the same choices when they have a souled breadth and depth of emotion to choose not to, it can still make sense that they don't feel fully removed from who they were. In fact, if they were totally separate in every way, the notion of any redemption being needed at all would be bizarre. In contrast, when Anya became human she showed little issue with her past, so her regret and change of heart about returning to being a vengeance demon in S7 stood out as distinct, meaningful progress.
But for when it could ever really be right to say any of them had achieved redemption is as you say subjective. It's too difficult to delineate. A lot comes from the inner meaning leading their choices I think, rather than just being grounded in the outward actions. We saw Spike progress further still in AtS 5, after his sacrifice in the hellmouth, when both he and Angel directly consider the nature of being heroic, the hope of reward and talk of the possibility of inevitable consequences for what they've done. Alongside feeling/knowing they were also victims once themselves, this progress feels more like a sense of deeper self reflection, rather than something attributable to 'bigger' or 'better' actions to make up for what they did in the past. For me, being willing to give everything of yourself and sacrifice attaining your own desires, giving up any/all other opportunities for yourself, for the benefit of others and/or the greater good of a world full of people you don't even know, is just huge. Whether that is reflected in your whole way of life and is repeated or is instead a single act, isn't as important as the underlying attitude and meaning behind the choice(s), the intent and selflessness perhaps. Yes it does fall to that subjective aspect, but that is where the probability of redemption lies for me. The character progression which takes the desire and want to atone and uses that to direct the choice to walk that path, to be on a journey, to want to make a meaningful difference to others
. That feels more significant to me as I'm not sure a quantity, an amount needed to 'balance the scales' as such, is really attainable.
