I monitored the situation during the day and so far the count of _sync docs in all buckets is around some thousands, that feels just fine.
Yesterday late afternoon, after I found the millions of _sync documents, I stopped the SG gateways and cleared all sync data in order to start from scratch:
- deleted all documents with meta().id like _sync% from all buckets
- removed all xattrs_sync data from all documents
- started SG, create dbs, set config, import function and sync function
As said above, so far it looks fine beside the temporary 1.619.167 _sync:unused as described in my post above.