The biggest challenge that comes with developing an rpg seems to be budget. If your team has the tools necessary to create the game, and aren’t turned away by the fact that they might not get much payment until the game is finished, then the only challenge is your commitment to making the game and seeing it through. I’m assuming you all have jobs given that you will be working on the game part time? With regards to your 5 acts, how long are they? Typically story RPGs have 3 acts, you may not want to stretch it out to 5, especially on your first run. What you have to remember though, is with these studios such as CD Projekt Red, Obsidian, Bethesda, Bioware, etc. They have to pay quite a handful of employees, and sometimes they have to outsource work to 1-2 extra studios in order to gain additional manpower. Then there’s the cost of the composer, voice actors, marketing, etc. You seem to have a pretty solid team already. What most indies have to realize is that they’re not going to be able to front the amount of money like developers who already have publishers. Now you can attempt to pitch your idea to one, but you should know that you may lose a bit of creative control over your IP. It’s a tradeoff. If your team is working in their free time on this project though, whatever money you can spare should be going to furthering the game. Things like website fees, copyright/trademark fees, software you may need to license, and such. If you manage to crowdfund your game, then you can start paying your actual employees, because you’d have money to give to them while still maintaining the upkeep fees for the game/company. Generally it can be difficult to get people to work for you for free until then, but if you’re able to show them that you have the skills and determination to see the project through, it will give them confidence that the project will be a success and they will want to be a part of it.
One thing I will say is that you should do plenty of research into the process of game development before you start this. You’ll need to know how to manage your team, a fully thought out game design doc, knowledge of legalities regarding your company, IP, employment, how you will market your product, and much much more. There are plenty of sites that can give you a head start throughout the web. Good luck.