Switch step 2 and 3 and you should be fine. You can't fully unassign a disk slot until you do the new config. If you complete these steps as written with the swap of step 2 and 3, parity will rebuild with the parity disks assigned at that point. Since you would rather build parity with the new drive, you would save a bunch of time by switching the parity drive out at the same time. Parity has to be rebuilt anyway when you remove a drive, so why do it twice.
So, stop the array, set the array to NOT auto start, power down, do the drive 7 and parity drive physical removal and add the 10TB parity, power up, set a new config keeping current assignments, go back to the main page, assign all the changed drives into their final slots, build parity. After parity is built, then you can do the drive replacements the normal way on the 2 and 3TB drives.