Hello you called for doujinshi?~
I really only specialize in Higurashi doujinshi, but I’ll do my best. I know I’ve seen RGD doujins being listed while looking at the lineups for 07th Party artists especially from S3 though I can’t think of ones at the moment I’m sorry. Though Pixiv is great for looking up artist line ups!
Now onto Higurashi:
Circle/artist: Cosmic Cosmic/ico
Known for the Higurashi/Umineko Motion Graphics art! You’ve probably watched them, if not then here’s the thread. Also worked on Umineko doujinshis.
I own Kono koe ga kareru kurai ni kimi o suki to ieba yokatta (Satoko-Centric) and just recently ordered ひぐらしがなかなかった 2 油蝉.
artist: Dobato (warning: nsfw)
Known for their contributions for the anthologies, rika-centric mostly. Worked on mostly the case anthologies, a few of those stories I’ve encountered also made it to their doujinshi. Spoilers for all of Higurashi in a lot of their doujinshis.
I own All about the Lyric (it spoils Higurashi in its entirety and some scenes are somewhat nsfw), ソラカケ (spoilers for all of Higurashi also has a crossover with Umineko), Sweet Star Candy (spoilers, mostly a gag doujinshi, and a few nsfw pages), and recently ordered 雛見沢虚構集.
I’d highly recommend getting “All about the lyric” doujinshi out of all their doujinshi. They made it readily has been available on pixiv in its entirety 4 years ago and Jorwil on tumblr translated it.
Special Mentions: unchhelix (Keiichi/Rika-centric), BAU (Keiichi/Satoshi, Keiichi/Satoko, Keiichi/Rika-centric), クロリカ学習帳 doujinshi (Keiichi/Rika, Keiichi/Mion-centric),
artist: Hibiki Mio
Same as above, but keiichi/rena-centric. They participated in the Higurashi Wonderful anthology and the Higanbana anthology manga as well as making some Umineko doujnshi.
For Umineko doujinshis I suggest these artists (don’t have any doujins by them sadly):
circle: Milktea (some covers have spoilers for umineko chiru)
known for being one of the popular fanartists back in the day
artist: Hanokage (one cover has spoilers for chiru)
same as above but has now become a mangaka with the release of the Madoka Magica manga, but also participated in anthologies.
circle/artist: MilkCandy/Suzushiro Kurumi (spoilers for chiru for some covers)
same as above, but also has become a mangaka with the Forgery of the Golden Witch manga.