You know that I wrote a
turtling guide and it conflicts with a lot of things that you said...
First off, turtling absolutely shouldn't be your goal. It can, at best, be considered a stepping stone until you are strong enough for "kill it before it kills you". Second, Def+ is a very worthless buff. Don't ever consider that, especially if you have access to damage cuts of any kind. The only one which will ever amount of anything is Joan's MEX, but even that one needs like 30 turns to ramp up to decent levels. Don't. Bother.
And why even turtle with Dark, when you can reach 25k+ HP with just a F2P Grid... ... oh wait, because newer players don't have access to a full Envy Grid. THANKS A LOT DEVS.
I am personally not a big fan of Ascension. The problem is it takes away from useful stats, for minor gains in a stat that will only be used by 1-2 characters in your party. Not great. CAN it work? Yes, but chances are high that you'll rather want something else if you can make that choice.
Also not a fan of Vigor, due to just how unreliable it is. Is it an amazing stat at full HP? Yes! Is it a good stat if you have lost ANY HP? Fuck no. Double skills are way better, when both stats are good stats. Assault, Pride and Defender are the best ones in general.