Synology SHR Volume - Migrate from 3 disks (RAID5-ish) volume into 2 new disks (RAID1-ish)

What I have: 3 x 6TB SHR Volume with 12 TB redundant storage (1 disk recently broken)

What I want: 2 x 18TB SHR volume with 18 TB redundant storage (I'll expand the volume after the migration)

Of course, I want to migrate everything, to avoid any additional work and downtime.

Details:

Today my Synology DS1515+ started beeping and one of the 6TB disks were dead. It's running a 12 TB SHR volume with BRTFS, which spans over 3 disks (1 disk. redundancy, like RAID5), so it's effectively running RAID5 on 3 disks. Now I've ordered two new 18 TB disks and want to run a 2-disk SHR volume with these disks.

To begin with, I'll just replace the broken 6TB disk, and one of the other 6TB disks, with the new 18 TB ones.

Then I have a 12 TB Volume which is still spread over 3 disks, but I want to get rid of the last disk.

But how can I then take the old 6TB disk out, and tell the storage manager to use more of the new 18 TB disks for storage, so my current 3x6TB redundant array with 12TB space, becomes a 2x18TB redundant array with 18 TB space?

I know that Synology uses LVM and MDADM under the hood but I don't want to mess around with this if I can do it the prescribed way, through synology's web interface.

Your plan looks so complex that I doubt Synology would have implemented support for something like this.

Trying to add a new unit to the system might trigger resilvering of the RAID array, which increases the probability of second drive failure and loss of all data.

The best option is to set up a new RAID somewhere else, and then restore data from backups. This has the least probability of data loss.

If backups are not available, then copy data from existing unit. Then you are risking failure of second drive and losing all data.