I'm approaching my enchantments a little differently. I bought 7s for all slots. Relatively cheap. With having to have 2 8s to get 9s and then 2 9s to get 10s, I've opted to go straight to buying 10s where that isn't an issue any longer. Is there a definite answer on which is a "better" route?
It all comes down to how much time you spend in game and what are you focusing on, if you have leadership at rank 21 where you can make Thaumaturgic Stone (3 a time around 3 sets a day if you didnt miss your timing), you clear Sharandar all quest daily (11 Thaumaturgic Stone), you can go ahead with the upgrade path. If you are good at earning AD, 2 Dungeon & 2 Skirmish a day, not missing Weekly 4500AD quest (6 total) & have a bunch of Rings to salvage around 10-15 a day? you can start saving them to buy a rank 12 straight away at 1.3m-1.4m each. I personally do both on both my main & alt, that is why i spend too much time on this game
Anyway just to let you know roughly how many Thaumaturgic Stones you need for 2xRP to get them rank up to rank 12, Rank7>8=7, Rank8>9=21, Rank9>10=63, Rank10>11=125 & Rank11>12=218, i personally would not upgrade a rank10>11>12 because the Success rate is 5% & 1% which you would need a coalecent ward for the 1% chance.
Prepare for more maths
Buying a Rank10 echantment is around 320-340k, going to rank11&12 you would need a total of 7 Superior Mark of Potency (70k each) & 1 Greater Mark of Power,Union & Stability. So lets only count Superior Mark of Potency, thats a total of 490k+320k(Rank10)+400k(coal ward)=1.21m and a total of 343(Thaumaturgic Stones) under 2x RP, without 2x RP thats 686 if you can get 20 a day that would take you 34-35 days not missing a single day. So heres the question for you guys, would you buy a rank12 enchantment for 1.3-1.4m or go for the upgrade route ?
Not to discourage anyone but i personally think Enchantment upgrade should come last, should focus on getting all your artifact & artifact equipment to epic(purple), then focus on 1 Artifact & 1 Artifact equipment at a time to Legendary, when there is a 2XRP you have Thaumaturgic Stones to spare, upgrade those rank8s to 9s, do that again in future when you have more to rank10 then stop, buy a rank12 then you would have a spare rank10 for sale or for your alt.