Sorry for barging in on a 5-year old thread, but I have been having this exact debate, and have also recently started trying out Sync.com for this purpose (client-side encryption) - storing sensitive financial docs, etc.
Curious if you ever landed on a solution? I see from the below that I may just be too insistent on putting these docs in the cloud. Perhaps local storage (backed up) is enough? I guess I don’t access these files too often, so not sure why I insist I need the convenience of cloud in this case…