Miyazaki was not involved in Grave of the Fireflies. It was written and directed by Isao Takahata.

For darker stuff pretty much any https://en.wikipedia.org/wiki/Yoshiaki_Kawajiri film will be good. Ninja Scroll is very violent but worth a watch for the animation at least.

https://en.wikipedia.org/wiki/Neo_Tokyo_(film) is a good 80s compilation.

Otomo has another nice anthology: https://en.wikipedia.org/wiki/Memories_(1995_film)

From Oshii related work - https://en.wikipedia.org/wiki/Jin-Roh:_The_Wolf_Brigade is fantastic (but slow moving except for a few places)

Since we're talking Oshii works, I feel obligated to bring up Patlabor 2: https://en.wikipedia.org/wiki/Patlabor_2:_The_Movie

It's a good example of extraordinary detail in design and animation around mechs/military vehicles, cityscapes, and retro UI designs, plus Oshii's proven directorial chops.

I also quite like the plot, which is an unexpectedly serious look at things like the JSDF's case for existence and the danger of military/intelligence bureaucracies to democratic governments.

The opening sequence, which these days looks super vaporwave: https://www.youtube.com/watch?v=IsMTpSDAGxM

Quite right, Patlabor 2 is fantastic as well!

It does go well together with synthwave: https://www.youtube.com/watch?v=redlketVxn4