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So I don't know of any Anime particular effects resources to be honest, but there are some amazing books I definitely recommend that show some incredible effects.

A great start would be finding Joseph Gilland's Elemental Magic effects book. The book contains literal stepped processes on many different effects animation composed by fantastic american animators, a lot coming from classic 2d Disney backgrounds. It's a great start, and it isn't very hard to find a free pdf of it online, or buying the book physically which for some reason I like more.

Amazon Link: https://www.amazon.com/Elemental-Magic-Special-Effects-Animation/dp/0240811631

Other then that, studying, and practicing what you find from great effects animators on Sakugaboru is what I'd recommend.
Some of my personal favorite guys that immediadly come to mind are:

Yoh Yoshinari (Classic Gainax/Trigger Effects)

Yutaka Nakamure (Impact Frames, fire, lightning, in bombastic style, the guy is a legend for a reason)

Masanobu Hiraoka (Surreal, and mind melting work, incredible what he pulls off)

Hope all this helps out. Have fun.

I don't know of any storyboarding websites/subreddits like you're talking about. I'd be interested in checking those out myself. I will say there are some cool anime storyboard books out there. The limited edition blu ray set of Gundam Thunderbolt from Japan includes the storyboards for all 4 episodes. Ghibli has a whole book series with the storyboard of all of their films and there was one released for FLCL recently. Mamoru Hosoda's films have some as well. I own the ones for FLCL and Princess Mononoke myself and its pretty cool to see them in such a raw form, especially the Miyazaki ones since he draws every single one himself.