I just finished Path of Radiance and kind of wanted to talk about the series.
Mine would probably be Blazing Blade or Mystery of the Emblem, BB because having some weird cult terrorist group as the villains was more fun than Evil Nation nº12837, MotE because i think it has some of the best gameplay and maps of the franchise while having a way better story than the other newer FE games (that i have played, Awakening and Fates).
iirc there are some things that you just don't find out about unless you play all the routes or look stuff up. and i think there's some major plot threads that just never come up in the blue lions.
i think all three houses feel pretty good to play, but i mostly agree with your assessments of the characters. something i find interesting is that blue lions feels like it was the intended main route for some of the reasons you specify there, and also because it's the most generic fantasy story of them all, but it wasn't.
spoiler
the first route designed was the church route, since the game started out with the idea of 'what if edelgard was your lord but then she betrayed you' and then i guess they decided they wanted to try and do fates but better. so the first act black eagles was actually the only way the game was going to be played originally, though obviously that was fairly early in the design cycle. but as a result both crimson flower and verdant wind feel a little underdeveloped for reasons i won't get into.
I'd be more fine with some plot threads not getting answered in every storyline if they wouldn't have kept teasing that we were gonna get answers for them