If you have a problem with him buying and freeing what slaves he can on the basis that the money just persists the trade, ...
First, nobody here said that there is only one degree of evil. Morality is a scale, with context and purposes possibly influencing judgement. You described some extreme cases without context and then put them in perspective with special cases. That's stupid. For two reasons: 1. absolute morality (i.e. an act is always good or evil regardless of context) isn't a thing, at least when discussing in good faith; and 2. these cases are not examples of things that are evil. You're just strawmaning here. (And I'm pretty sure you tried that before with the "trapped inside" example, using a fire as additional context then.)
But, as much as you want to pretend that slavery is less evil when you treat your slaves humanely, it is still evil to some extent. I won't go back over the reasons why, I already made a post about this. It's even worse in fantasy worlds like this where slavery is often enforced by magic, completely removing any free will from the equation and solidifying the slaves' status as property.
Second, from the quote I kept above, that's not something that was ever discussed here, for all I can remember. (I might have missed something after I got tired of the discussion.) What was discussed was the idea of buying them, then bringing them along in the MC's travels while treating them humanely... but
still enslaved .
Freeing them was not discussed. Worse yet, the argument was to buy the best slaves who would be treated better even by other owners than the useless or injured ones. Because they could contribute to the MC's fighting or survival needs.
I would be more in favor of this new behavior, and I wouldn't call it evil if that was the idea. I'd still find it useless in practice, and potentially still harmful in a broader scope, but at least it would not be evil because it would end the bought individuals' status as slaves.
And I think the reason it wasn't brought up is because we were comparing this MC to the ones in other mangas where isekai'ed MCs buy slaves without feeling guilt. Because they're thinking of themselves as heroes who are (not really) freeing poor slaves from a life of hardships... while bringing them along into repeated life or death situations.
The comparison was not to the (few) MCs fighting to end slavery in their new world. I can think of a few, and none of them are going at it with "I'll buy one slave once in a while" approach. They are either OP and go at this with a frontal approach because they can fight the whole world if it comes down to it; or they have a long-term reasoned approach that aims for a societal change. Sora doesn't have the individual power to force his moral choices on the whole country, nor the personality to work politically towards the required change. Either of these two points might change over time, but right now that is definitely outside the scope of his power or behavior.