ElasticSearch sync two clusters

Hi all,
If anybody could help me i would be very grateful. Problem i have is next:
I have one cluster with 5 instances that's operational. It has over 10 TB of data and it creates snapshot every day. I created second cluster that's main purpose is disaster recovery. I managed to restore snapshot from original cluster without any problems. When i try to restore new snapshot on replica cluster it starts restoring from beginning and that takes a lot of time (aprox. 10 hours).
Original cluster is creating snapshot to s3 bucket, and replica is restoring from the same bucket. Replica has read-only rights on that bucket. Replica doesn't create snapshot.
Is there any way to have those two clusters in sync without big downtime on replica cluster?

yes, see cross-cluster replication

I saw this, and the only reason i haven't tried it is because the replica cluster version is 7.5 and the old one is 6.3. In prerequisites table says it's not compatible. Is there maybe a workaround?

I'm not aware of any workaround. Ideally you should upgrade the 6.3 cluster to 6.8 (which contains many bug fixes).