the code for SHR is still there and active, synology only diabled the ability to create a SHR RAID
so if you install a new *.pat (~200MB, not the small update files) like 6.1.1->6.1.2->6.1.3 your SHR in place will still work and you just loose the ability to create new SHR's
you write you dont have data on the disks so then just delete the raid6 array and after aktivation of SHR you crate a SHR RAID (SHR1 one disk can fail, SHR2 two disks can fail, difference is like raid 5 to raid6)
if you create a SHR1 it might create a RAID5 volume of 6 disks 1.82TB + raid5 of 3 disks (the 3 bigger then 1.82TB with the size 2.73-1.82TB + RAID1 of 2 disks 3.64-2.73TB
so usable space will be 5 x 1.82TB + 2 x 0.91 + 1 x 0.91 = 11.83 TB
if SHR2 every position in that list looses one disks so it will be
4 x 1.82 + 1 x 0.91 + 0 x 0.91 = 8.19 TB