Figured I might as well give my impressions of the complete game, now that I've finished it. It was...
Just really, really awful. A 3/10 if you want a number (whereas the original Valkyria would be a 9.5, the second a 7.5, and the third an 8.5). Divorced from those three and judged on its own merits (which is the only fair way to try talk about it imo, seeing as the setting and systems are completely different), it's best described as a super janky, padded mess.
Padded in two ways - with copy-pasted maps and missions, and with hilariously loooooong cutscenes (where nobody ever shuts the fuck up, and nothing that couldn't have been said in just two minutes is ever uttered) that feature the worst staging and composition I've seen in maybe a decade.
These cutscenes also inexplicably fade to black... only for the next one to start... and it's the same scene? And no time has passed? Huh? To whoever directed the cutscenes - I can't believe you've done this.
The story they're telling isn't great either. It's not quite bad, but it over-relies on stupidity and over-exposes the boring villains by constantly cutting to them in their lair, where they all wear scary armour / hoods and deliver inane speeches at one another. The setting is a huge step-down from the previous games, taking place in a more generic anime-fantasy world where everyone can use magic, which really undercuts the titular Valkyria (who is a laughably bad character / does a fine job undercutting herself by losing to you a half-dozen times / lol see what I did there with the 'tit-ular'?).
The playable cast is fine. There's a faux social-link system which was one of the few things I unironically enjoyed. The main character is about as fun as a wet blanket; all dour and brooding and edgy. Everyone's designs are uniformly terrible.
I've held off on mentioning the actual gameplay because I'm not sure what to even say. It's very janky, and feels super dissatisfying to control because of the cool-down every action triggers (like a global cool-down in an MMO, except your auto-attack is bound by it too). Strategy / competence means very little in comparison to whatever level you are versus whatever level the enemy is. There are a ton of party members to play as, but the system renders them all equally uninteresting. Guns suck.
As a silver lining, you can fill up a gauge at the top of the screen which in turn disables the action cool-down, and lets you play a semi-decent ARPG for a bit. The upgrade systems are pretty decent too.
The music is fine. Kinda disappointing when you consider it's Mitsuda. The look of the game is sub-par, with rubbish animations and a ton of distracting pop in. The oddly-detailed character models clash with the more pleasing (if overwhelmingly brown) world around them, and overall, it just looks worse than the game from 2008.
The mission padding strikes when you least expect (I actually thought I was about to finish the game, only to discover I was just halfway through (at which point I dropped it for two weeks)).
To be completely honest, I wouldn't recommend this to anyone.
EDIT: The long-ass, slow cutscenes were pretty good Japanese practice though.