Hello and welcome to RaNGE! I see that you're working on a large-scale Touhou fan game with 6 Stages + Extra + multiple routes.
As a rather longtime resident here who has worked on multiple projects and large-scale games, I'd like to share my 2 cents.
Firstly, I see that you are planning for your assets, including music and art. Do note that creating these assets takes a lot of time, especially given the sheer number of characters and stages you have. In general, making cutin images isn't too hard for artists (I assume you are capable of making your own cutins and boss/player graphics since you're coming from dA), but creating and animating sprites takes a lot of time. If you're not familiar with sprite sheets, I suggest that you start working with those. As for music, music is a very important part of the experience and you will want to ensure that the tempo and feel of the music work well with the stage/boss.
As of the time of writing, you have not specified which engine you are planning to use. If you decide to use Touhou Danmakufu ph3, we here at RaNGE have a number of people who can assist you. There are also quite a few resources. However, if this is your first Touhou Danmakufu project, I highly highly recommend that you do small script first to gain experience with the engine and how to code well, especially in regards to writing well-structured and refactor-friendly code, creating fun and enjoyable danmaku patterns, and providing a good user experience to the player.
Finally, before you start any real work on the project, I estimate that a project of this size will take about 2-3 years to make for an experienced coder and more for less experienced coders. Assuming your entire team works well together, you could bring the time frame down significantly, but this requires constant communication, a well-organized workflow, and good project planning + milestones + actual deadlines for when you want to have tasks done by.
For example, once I have a stage planned out, I can theoretically complete the code for the entire stage in about a week, but it usually takes 1-2 months due to various problems that arise, as well as life in general taking up time.
Anyways, good luck! Make sure to check the stickied posts for resources to help you on the project!