IMO the spanish and portugese where always less obsessed with race mixing than the eglish and other europeans.Then they'd replace it with something that is slavery in all but name, which was in fact what the Spanish did with their encomienda system or the forced labour the Jesuits used in their missions. It could probably even work the same way, where the not!slave traders go to Africa and buy the freedom of slaves on the condition they become indentured servants to pay off their debts. I can imagine the vague prospect of freedom sounds more appealing than a life of slavery, so they'd find plenty of volunteers. And to the African powers engaged in the slave trade, they'd act just as OTL to ensure there was always a large number of slaves ready for sale.
Reason being is that slavery was too highly demanded and too profitable in brutal operations like tropical sugar production and mining once the native population collapsed in part due to the brutality of said mining and plantation agriculture. If the Catholic Church bans slavery, then it will be replaced with something else lest Protestant states like England or the Netherlands gain an advantage.
The one interesting factor is that even if slaves are coerced into signing contracts for lifetime indenture, it would be harder to coerce their children into that same system. That would have a huge effect on the African diaspora in the New World where there'd be way more free blacks. And would the Spanish/Portuguese/French/whoever be importing as many women? Would there be more mixing between black and indigenous populations?
Further though I agree with most of what you say I think there would be legal differences. The not!slaves would have at least some rights. The encomienda system was at least in theory placing also requirements on the holder of it. Also the system would be much easier to abolish when finally the time came for it as the not!slaves would not be considered the property of their not!master. I think its important that these people would by every side and by the law considered as people - human beings and not the property of someone else. With the greed still there but absent that level of dehumanization their lot though still horrible might be better compared to OTL.
What I mean is that at first glance the differences might not amount to much but there would be a lot of subtle difference that might be important later. Also I think especially children of spanish colonials and not!slaves might have a much better chance to become free man.