Please, step into my parlor of unsolicited advice for a moment...
will eventually be an open source project.
Lean Software people are right about one thing — you've gotta get software in front of users as quickly as possible. If the project timeline is "work for 6-8 months, then show people" it's possible you built an amazing thing only you know how to use and nobody else really wants.
Release early, release often.
My problem is that I have a full time job. When I come home at night
The simplest hack here: wake up earlier and use your first (and best) 2-3 productive brain hours on your project then go to work. You'll use your best performance (refreshed, alert, not tired) for your project while still not being fired.
I've considered quitting my job and diving into it for a few months. I have savings but this seems foolishly risky to me.
Yeah, that seldom works. Practical advice is: don't quit a job unless your side project is demanding an increasing amount of time and resources from customers/users (meaning: it's becoming successful). Non-practical advise is: screw work, if you're not happy, save money and do what you want.
If anyone has been in a similar situation, I would greatly appreciate advice.
The world isn't HN. You won't be an overnight success. You will fail. Nobody will use what you make. Or, you could be the next person the media starts comparing others against as a measure of unexpected success: "Is this new kid the next bglazer?"