Ceph is the only viable alternative. To do it properly, however, you must get rid of ZFS. Ceph has its own native filesystem and they actively (maybe they even refuse it now) discourage the use of anything else. This would be an entirely different product altogether since ZFS is no longer in the equation.
Sure, there is lustre and lizardfs and moosefs but those have glaringly obvious caveats. (Paid for, small development teams, haven’t been around for a long time, murky corporate “ownership”, etc)