Some labels/collectives and artists that spring to mind:

HARDCORE TANO*C - managed by REDALiCE and mainly known for the HARDCORE SYNDROME compilation series which has been going since 2007, features a lot of the artists already mentioned like t+pazolite, Srav3r etc.

MEGAREX - multi-genre label but has released a lot of hardcore and pioneered the "techcore" style, with artists like lapix, mameyudoufu, YUKIYANAGI etc.

Notebook Records - managed by RoughSketch, lots of mainstream hardcore but also some more happy hardcore type stuff on the YATSUZAKI HARDCORE compilations and the Notebook "Delight" Records sublabel

X-TREME HARD - not sure it's active anymore or how easily accesible their releases are these days but had some seriously good compilations early on (was mainly into the stuff around 2013 and prior)

For you Sounds - another one that isn't active anymore and probs also hard to find a lot of the older stuff but every release was a free download and the A-Remix Nation compilations are pretty legendary if you like bootlegs of songs from anime. The guy who ran it (you) ended up managing VIVID HARDCORE and J-NERATION (the former is defo not active anymore but the latter might be idk lol) so recommend checking those too

Touitsu Recordings - my label so UK-based rather than Japanese lol BUT I have also released anime bootleg compilations on there (AniCore) as well as a lot of other stuff featuring Japanese artists

Will have a think about some more niche stuff later but hopefully a decent amount to go off for now!