Well, you get at least some points for considering my concerns, and not getting all defensive.
My core concern is not that you have a 3D artist on board, but unless that person is as/ more dedicated as you, then you have a huge problem.
To make it short:
How will you lead a 3D game development team, though your background is all in 2D?
Unless you intend to seriously expand your skills to 3D, I'd strongly suggest you re-evaluate your game plan.
By the way it sounds now, you don't have a core skill to the project you envision.
As said above, To get a game of any kind done, you need competent and qualified leadership in Art, Code, and administration. People, or a person who can guide and teach others in the craft, and point others in the direction they want, and if there isn't proper personnel, do the job themselves, or be able to recruit qualified people.
No offense, but from what you've described and admitted to me so far, this doesn't meat the requirements and definitions of "serious" or "Startup"
If you can crank out and organize a proper GDD, you might be able to fill the administrative branch, but as you've presented it to me, you unfortunately don't fit the Artist pillar for the project you've described.
It sounds like your laboring under the impression that a designer just comes up with the ideas, and others make it happen. That doesn't happen anywhere.
Don't feel too bad, I've found loads of people from all ages and backgrounds as or more oblivious when it comes to game dev.
Put it another way,
Why would the 3D artist follow your directions if it is clear you don't know the 1st thing about critiquing 3D art?
How will you determine proper task assignments and schedules if you have no idea how long making a given asset is spose to take?
Why would the programmers listen to making your mechanics, when it's clear you don't know the basics of why this engine is more suitable for the design and better than other engine options?
Why would the writers listen to you when it's clear your not sure what you want the story to look like?
How is the composer meant to give you a seamless audio experience when all you have is some mechanic ideas? but no themes?
I could go on....these are rhetorical questions.
This post will expire in 30 days from when you posted it, and instead of the natural instinct to delete all this critical critique to make your recruitment look better, allow this kind of feedback to help guide you in the direction you need to go in order to maybe one day have the project you dream of.
A while ago, I came across an audio composer who wanted to spin up his own project, and be project lead. I responded to him, and while I did so,I began to think, why I haven't heard of many or any projects with a composer as project lead. I realized the same thing I'm telling you now, that your background and skill set is not a core skill. I also asked my lead audio why this might be, and we had a long discussion on it, which we came up with a similar but another critical reason. And that reason was because Audio usually comes at the end of the development pipeline, and doesn't usually directly interact with art and code in critical ways. Meaning, it's not part of an audio person's job to know the art or code pipeline, thus they can't properly help problem solve those issues.
Your skill comes in preproduction, and maybe a bit on and off during production, and also at the end of production/ post production for promotional purposes under PR. It does not directly interact with 3D art or the code pipeline, thus it will naturally be harder for you to manage or oversee those departments, even indirectly.
If you really want what you want, you'll need to learn new skills, and rise to the occasion.
If you don't want to or can't do it, this isn't the thing for you.
(And if it wasn't clear, you did not answer the above questions in a way that inspires any kind of serious recruitment. You have an idea, come back when you have a GDD and the foundations of a project. (Or be honest about where you are, cutting all the fluff) only then will you have the chance of finding what your looking for.)
*or enough money to pay people not to care about your shortcomings*