But Christ himself said that he didn't come to abolish the law and that not one jot or tittle would be changed until all is fulfilled. Galatians is talking about relying on works of the flesh (including the law) for salvation. It is not, IMO, teaching that the law is bad for you and has been abolished.
The fact that keeping the law is not a condition of your salvation doesn't mean that keeping the law and tithing isn't a good thing to do. After all, sin is simply 'missing the mark' and there are sins of commission and sins of omission.
Do you think that, at the judgment, God will tell those who tithed, "Well, that was nice but since it wasn't a NT requirement it doesn't count."? Since you spend a large part of your life earning a living, when you tithe you are giving part of your life to God.
"However, I can't think of a single Christian sin that isn't enumerated in the NT."
It does seem that you are looking for a new set of laws to keep in the NT. Perhaps Galatians does apply here, albeit in an unintended way.
And yet Paul circumcised Timothy to spread the gospel. Was Paul 'sinning' when he did that? Does it occur to you that it may have been a sin NOT to circumcise Timothy in that situation?
Likewise, could it not also be a sin NOT to tithe if God calls a believer to do so to spread the gospel?
He who has ears let him hear.