Recover/resume volume sync after system restart when device was unresponsive

Support wasn't very helpful and directed me here. Hope you guys can help.

Have a ReadyNAS 626x (FW: 6.10.8) and was just finishing up on upgrading three of my disk drives in the system. 6x8TB -> 3x18TB + 3x8TB with X-Raid enabled. First two drives replaced and synced no issues. Last one was syncing fine and was about 9hrs or so away from completing. NAS was still in use and was trying to organize files as this was going on. Mostly moving large files to directories and renaming as well as other background apps and processes.

Problem occurred late Thursday night last week when doing some file renames in a Windows explorer window, the NAS server froze up and never returned. This actually happens occasionally but it comes back after a couple of seconds (seemed like a disk was spinning up). Well this time the NAS never got back up. It could not be seen on the network and it was completely inaccessible. Regrettably, I had the bright idea to restart it thinking it would just continue resuming. Did a quick search here and came to the conclusion that it would. None of the buttons on the NAS server were responding at all so hard powered off by holding the power button. Turning back on, the volumes didn't mount and now everything is down.

Not sure how to move forward from here. Unfortunately, I do not have an up to date backup (yes yes, lesson learned). Have a couple of 16TB externals for backup but halted a couple of months ago as the backups weren't balanced and needed to be reconfigured.  What's on the backup drives now is about 15TB of ~35TB used

Can this process be resumed with no issues or is data loss inevitable?

We can be fine with losing a couple days of data, but it would be devastating if everything was lost. All the disks are fine, just had this interruption during sync. My Linux knowledge is limited but functional. Can provide more info as needed.

Thanks