I am at a difficult position to answer your question, as I have never used Ditto in sync with several computers and I don’t know Spartan.

Ditto has a “Backup Database” and “Restore Database” feature. You can backup the database to OneDrive and restore it on your other PCs, as you did with Spartan.

You can also have it done automatically over the internet. I understand you having trouble trusting it to send your data over the wire. However, if I may vouch for it, there are two things that make Ditto trustworthy: it encrypts everything with a password of your choosing and it is an independent opensource project. This means there is no big company behind it and that you can, with some programming knowledge, inspect its entire source for any suspicious behavior.