This, Lucia Route is my most favorite route in the game as well. I believe that you nailed it, the thing that makes Lucia Route the most facsinating is its balances. The romance aspect between Kotarou kun and Lucia is taken care of, Lucia's tsundere lovey dovey and their romantic date and kiss, the action aspect is taken care of in the fights between Lucia and Shizuru, and Lucia and Kotarou, the interaction between the heroines aspect is taken care of between the eating parfait between Chihaya and Lucia and their fight,(which I believe is a very emotional scene) and Chihaya also appears here, and the Gaia Guardian aspect taken care off with the scenes about Guardian and its Catholic bent. Also, there is also that scene with Lucia being best meido ~. There's also horror, and uh, lewdness (Not, not really).
I believe that Lucia Route is the most-written and the most balanced route in all of Rewrite, since it seems to not lack in anything and is well-balanced in all of its elements.
Regarding Lucia, she's a classic tsundere, turning from the Iron Fist of Justice to kawaii moe character. It's quite obvious in the Common Route that she's like that, she shows her more cute and embarassing aspects from time to time, then goes to tsundere mode again. I notice it in Lucia Route in the scene with Kotarou in the bus and her meido scene that she's like that.
Dunno, but whatever their workflow is, they sure ensure that all of their characters look cute and attractive. Key Visual Arts characters always have this distinct and unique personality, identity, artstyle and appearance to them, whoever the studio adapts the visual novel or whoever author wrote the visual novel in the 1st place, starting from Kanon, Air and Clannad to Rewrite, Angel Beats! and Charlotte. Maybe it's because the same people designed them?