Yes, multi-way sync is a nightmare even for a dedicated sync tool. I would only consider a star approach with multiple devices syncing back to 1 master with no overlaps. Or the master pushing out to all slaves.
If you really need multi-way, multi-device sync, consider syncing to separate folders on the master and then having a periodic reconciliation process to automatically remove inconsistent files where you can and report where you need to do a manual reconciliation otherwise.
Alternatively, cough up for more space. I got grandfathered into more space on my Microsoft account so I tend to use that. I also pay for 1TB of Amazon storage since I already get unlimited photo storage via Prime. My NAS will back up files (pre-encrypted) to Amazon. So for me, everything goes back to the NAS and the NAS gets backed up. Office 365 Home or whatever they currently call it is also good value, you can get up to 4 TB off a single subscription by creating sub-accounts. 1TB per account.
