(I can't reply to the messages for some reasons, thus, I will just post this way)
Discord server would be very convenient. Even if it won't be big, it sure will be fun.
I did hear that new Artificer have some legal issues. Keep forgetting.
Seija items are very interesting considering it might be hard to implement in DnD world. For example, Gap Youkai Umbrella. How are you going to cross the screen in DnD? Cross to the other side of the map on the table? Walk through solid walls? Just teleport as a bonus action? It sure will be tricky.
I think the hardest thing will be to create spell cards for each character. It would require to know how every single spell in the entire game works! As well as balancing been wacky. It's quite hard to fit all Touhou 6 characters in 1 short campaign due to characters leveling up very slow. Thus, leaving 7th level party fighting CR 27 Flandre. Whilst you can encounter those characters out of specific conditions because DnD is an open world game. The spell card rules might help with this. Considering they are made to equal everybody. Maybe implementing more of those would be an interesting idea. Considering that the rules of spell card battles is quite unclear in the addon. I might have hard time explaining them to my friends who are unfamiliar with Touhou.
Aside from all else. I think the best thing I can only suggest is some sort of the map for players to navigate in the world. There aren't many good maps, maybe making our own would be an interesting idea. Considering that's what I am currently struggling the most with preparing for my future campaign. Currently trying to figure out the map for my oneshot because it is quite unclear what's in the way from Moriya shrine to the Hakurei shrine.
I will probably finalize my campaign based on this addon, maybe throw in some homebrew items and then I will probably send it to you with notes of what my players did. Maybe it will help you make your own adventure to go with the addon. Although I don't say that you have to. After all, seems like you don't have many hands to work on this masterpiece.
Well anyway good luck with developing 1.2V, take your time.
Considering how slow my table is in finishing current campaign. I think I can wait about half a year. Maybe more. Take as much time as you can to polish the upcoming masterpiece. This addon already goes in hand of "Touhou D20" for DnD 3.5e if not already been better.