The only remaining question is regarding my raid groups.
I was initially planning on adding 2 disks to the existing 18+2 raid group to make it a 22 disk raid group and then add the additional 22 disks to the aggregate as a second 20+2 raid group. This would leave me with a 44 disk aggregate consisting of two 22+2 raid groups.
My concern is by adding those 2 disks to the existing 18+2 rg, will that create an imbalance/hot disks? Would it be just as good to leave the 18+2 rg alone and simply add a new 22+2 rg to the aggregate?
I will be running reallocation jobs on all the volumes after.
Match your RG as best as possible. a difference of 1 or 2 disks rarely makes a notable difference, but anything more than that I would adjust to they are close to the same, if not the same.
If you have enough spares, I would create 2x 22 disk RGs. Any "hotspots" should be taken care of after the reallocate is done. It will move the blocks over any new disks added. Even if you don't run the reallocate, WAFL will eventually use the new disks for writing date. the reallocate just speeds up the re-distribution of blocks to new disks in the aggr.