Any time you need to renew your Mesmer skin just roll and hit 2.

You can use Hardened Wellspring (+20%Str) from the Zenurik school with Molt [Vigor] (+45%) for a 65% Str boost at any time putting you at 304 Str if [Growing Power] is up. If you have 250+ kills in that mission maxing out Molt Augment then that's another 60 Str putting you at 364 Str. His 4th ability is a nuke with this kind of Str.

Plenty of Duration to make Thralls usable.

Throw in [Enemy Sense] for doing the Index since I can't imagine why anyone wouldn't be running [Primed Animal Instinct] if they have it for everything else.

Auger Message over [Constitution] because if you are paying attention to your Mesmer Skin then you won't be getting knocked down or staggered, so might as well take the overshields.

Thanks to this build being in the positive on all stats just about any subsumed ability should work just fine. [Stretch] is mostly for Subsume abilities, but you want to run Vanilla you could swap it out for whatever. Since it doesn't affect his 2nd or 4th ability and isn't going to do anything noticeable on his 1st or 3rd either. I would suggest a quality of life, survivability, or more Duration for your thralls depending on how you play. You already have an overkill amount of Str and won't really need any more Efficiency.