The earlygame has some pretty long boss-less sections of navigating the map, Winlan Cave, etc... I don't think boss density drops off, and there are certainly several multi-boss dungeons later on (Wisdon, Lab, and the Obelisk). For another way of measuring the game... one guide lists the dragon shrine as 33 of 109 in a walkthrough, another after 4 of 15. If there are dragons on floor 2 it's only a tiny fraction, and for me that's enough of a shift in the game to assign it to a floor break, and 2/7 of the game is a much better fit than 1/7.
Agreed that Deis is frail upon joining (which I'd peg as F4). And honestly she doesn't really come into her all-around goodness until F6 looking at Meeple's notes... she's still lowering the HP average as of Cort and running a 110 damage spell as her best, it's not until Mothro that she's flinging BoltX and presumably has good HP. There's not too much game left after Mothro... certainly less than precedes the first dragon forms, so yeah, we're looking at around two floors here.