Move old backup to new NAS double the space reqierement now!

Good morning!

My problem is i bought a new NAS with double size and try to move the old backup data to the new NAS. Unfortunately the new iSCSI block device cant be formated with 512 byte block size:
mkfs.ext4 -b 512 /dev/sdb1
mkfs.ext4: invalid block size - 512

Can only format it with 1024 B blocksize!

So now the backup data needs double size in space, my old is using:
/dev/sda1 32T 28T 3,2T 90% /srv/urbackup

My new copy with ext4 fs did not fit into 45 TB disc. Mynext try was i did format it with ZFS, turned compression on and got:
zstore/urbackup 45T 42T 3,7T 92%
But rsync is still not finished! I think it will be full even with 30% compress ratio:
zstore/urbackup used 41.2T -
zstore/urbackup available 3.61T -
zstore/urbackup referenced 41.2T -
zstore/urbackup compressratio 1.31x

So what is the best way do move to a bigger NAS? Right now it makes no sense to buy a bigger NAS if the data double after moving!

I suspect the backup links are now 1k and had been 512bytes before? Thats why it needs so much more space?

I dont know does urbackup switch on zfs features if i only rsync the data from ext4 to zfs and start urbackup with the new target disc? Or has the first backup to be special to start using subvolumes?

Will btrfs be the better choice? Is it realy ready for production environment?
Did somebody already switched from smaler NAS to a bigger one? The new NAS has 8 x 16TB HD’s as a raid 6 and do export a few iscsi block devices.

Any hints?

Exactly what i tought, 28 T do not fit into the new NAS with 45 TB space!

/dev/sda1 32T 28T 3,2T 90% /srv/urbackup

zstore 128K 128K 0 100% /zstore
zstore/urbackup 45T 45T 0 100% /zstore/urbackup

Makes me speachless! Alternatives?