Capacity calculation and planning using X-RAID2 with Dual redundancy

I have a RedayNAS Ultra 6 Plus that I initially configured with 6 2TB drives X-RAID2 and Dual redundancy. This gave me 8TB of space (which I never took the time to even try and understand why).

Recently I started getting SMART+ warnings about one of those drives and decided to replace it.
I remembered this animation of extending X-RAID2 and because My drive was already at 87% I decided to buy not one but two new 3TB drives expecting my capacity to grow by an additional 1TB.

As it turns out this was not the case.

At first I thought there was an issue with the expansion process not being triggered triggering so I tried to check the check the box titled "Check and fix quotas on next boot. This process can take several minutes to more than an hour depending on disk capacity and the number of files on your volume." as was recommended somewhere but this also did not work.

finally reading through the forums I found the following explanation:

X-RAID2 dual-redundancy uses RAID-6 layers. RAID-6 layers require a minimum of four disks.

So how do you calculate capacity for a given set of hard drives configured for X-RAID2 DR (Dual redundency) ?

first of all - you MUST have 4 drives to use Dual redundancy.
now, look at the smallest drive and suppose it has size S1.
count how many drives have at least S1 (lets call this N1).
if N1>=4 then add to expected total capacity S1*(N1-2).
now repeat with "un-accounted-for space" on all drives.

e.g.
Suppose you have the following drives 1TB , 1TB , 2TB , 2TB, 3TB , 4TB
The smallest one would be 1TB and we have 6 drives with that "un-accounted-for" space on them,
so 1TB*(6-2) yields 4TB and leaves you with 0TB,0TB,1TB,1TB,2TB,3TB "un-accounted-for" space.
next we consider another 1TB layer on the last 4 drives  that yields 1TB*(4-2) = 2TB more and leaves you with 0TB,0TB,0TB,0TB,1TB,2TB "un-accounted-for" space.

So what if you now want to get more space? the first layer is "full" (i.e. it uses 1TB of all 6 drives) but the second layer can be expanded in several different ways with the minimal one being just expanding one of the first two drives by an additional 1TB  and thuse getting 1TB*(5-2) = 3TB for that layer.

This was about calculating... you should now be able to do some planning by your self.

As for me, after understanding the situation I am at 2TBx4+3TBx2 yielding  8TB I thing the future expansion path would be by first replacing two more of the 2TB drives to 4TB each, this will give me an additional 2TB (try to calculate it yourself - 2TB,2TB,3TB,3TB,4TB,4TB) - then continue like this to each time replace the small pair to become the new largest pair.

The advantage is that from now on I will only need to replace two HDs each time to get additional space.
The disadvantage is that I some of the additional space I pay for will only be used when I buy the next pair of HDs.