This makes me realize that unless Stance Change is modified, Aegislash is one of the few Pokémon in which Dynamaxing adds limitations, as it wouldn't be able to change to Shield Form for 3 turns.
Is that really a major hindrance, though? If you want Aegislash to defend itself when Dynamaxed than use Max Guard. Otherwise I'd imagine you'd be attacking as that's all a Dynamaxed Pokemon can do outside of Max Guard which all Status moves turn into. Also you could probably Dynamaxed while still in Shield Forme, though if being able to switch between the two Formes is part of your strategy than maybe Aegislash shouldn't be you Dynamaxed Pokemon. If it's suggested movesets are anything to go by on its Smogon Strategy Pokedex, it would rather stay small to take advantage of King's Shield and than throwing around Toxic or setting up additional defenses than going all in as I'd imagine a lot of Dynamaxed Pokemon would be (and there would probably be a batch of defensive-focused Pokemon better for Dynamaxing as their stat layout will stay the same).