Is iCloud Legacy as good as it gets as a sync service currently- or as close to good as any of the options get, or is their a clear best service I should consider?
See this post regarding sync options and considerations…
That being said, Legacy is a different mechanism and seems more robust at this time. However, there are people who aren’t having any CloudKit issues. I’d suggest Legacy over CloudKit at this time. (Actually I’d suggest a sync on the local network unless a remote option was needed.)