OkuuFallout
Onshi
There is another Touhou forum website called Maidens of the Kaleidoscope. These two websites are similar, although the other is much less active after February 2020 when they had to move sites. Moriya is more for the downloads and the interaction of the community, while MotK is more for the technical stuff/resources and for posting your own original content (namely the RP threads).*
They have a specific board for people wanting to make their own danmaku games amazingly named Rika and Nitori's Garage Experiments (RaNGE). Aside from the humor value of the name, it has excellent sources for creating your own danmaku games.
One thing I will say, however, is that the site looks like it's from the early 2000's, when it really isn't. It just uses a format called Simple Machines that looks to be outdated, yet it is still being updated and patched to this day.
Here is the link https://www.shrinemaiden.com/index.php?board=5.0
*This is just my opinion from what I have gathered. I have been following MotK longer than this site, and I still don't know too much about it. I'm only including this footnote because I don't want any issues/drama to start.
OkuuFallout
Onshi
Personally, I really don't know much about it, I only did the basic research. But based on what many other people have said, Danmakufu is really good. But in the post I am linking below, it does have recommendations for other resources, albeit somewhat limited.
https://www.shrinemaiden.com/index.php?topic=129.0