To be fair, the Soul of the Colossus can be forged into the Soul of Dimensions, encompassing five total "Souls" with their own unique benefits and accessories as well as infinite flight, 30% damage reduction (as opposed to 20%), a huge +500 HP, as well as replacing your Elysian Treads as opposed to taking up their own individual slot.
Therefore, the extra grind is worth it. 100% worth it. It's like getting your Ankh Shield. It's painful and takes forever, but it pays off in the long run.
Very interesting possibilities. Might I suggest you incorporate the Black Belt effect in the Super Sonic Soul and the Soul of Dimensions, given the Master Ninja Gear is a component? It'd be ridiculously handy even if you don't add the modded accessories.
EDIT
Oh yeah, it'd also be amazing if you could include the Waifus in the Soul of the Universe and be able to toggle them with the vanity. Is it possible for you to keep their damage scaling effect, given that's one of their main benefits?