Oddly, the one that seems to get close to my desired appearance is the 1000x160. I have no idea why I tried those image bounds at some point, but several of the recent albums have worked with that. You should see what I got when I used those image bounds and then chose fixed shape!
Oh, yes, that would be a disaster. It would, of course, make every thumbnail precisely 1000x160px, which would produce a strange result, indeed. "Fixed shape" really does mean "fixed shape!" But for justified thumbnails, the wider image bounds are better at handling originals that have more extreme aspect ratios, e.g., something more than 2:1.
Now that Neptune allows fixed shape thumbnails, I've changed the default bounds to 300x200, precisely to avoid that unpleasant outcome. The release notes warn about it, in fact.
I've chosen 300x200 because the 3:2 aspect ratio matches a lot of digital images, so it's a good "most cases" choice.
I've gradually started gravitating to larger thumbnails. As long as you've got a responsive layout, there's no huge penalty to letting them get a bit bigger. And bandwidth isn't as much of a concern as it used to be. In fact, I tend to use "high DPI" thumbnails more often - the image files are larger, but they really do look better.
My own family album uses Pluto (same thumbnail layout as Neptune), justified thumbnails, bounds of 330x220, high DPI, and folder thumbnails at 140% height. But my public "travels" album uses Neptune, justified, 1000x160, non-high DPI, and folder thumbnails at 150% height. But I'm probably going to bump some of that up a notch.