Ask HN: How to quit a side job that became too much work?

I've spent the past two years as a solo sub-contract developer building a custom ERP system. It's become too much work for me to do on my own and side work no longer fits into my work/life balance (now with 2 kids in 2 years). The project started off with minimal requirements and sounded like a 3-4 month project. I've been working on it around my day job, around 10 hours per week. It seems to be in and endless "beta test" cycle and constantly changing requirements without before even going live. Eventually it's going to be a live system with over a hundred people using it for their daily operations. I was not aware of this at the time. I have no interest in maintaining it and need to find a way to get out. I am working without a contract. I submit my invoices to another contractor who then submits his time and presumably my time with any applicable markup. I bill by the hour for tasks. I don't have a direct relationship with the end client. I've been doing side projects for the main contractor for the past 4 years. I have never signed anything other than an NDA about 4 years ago. I mentioned to the main contractor that I don't want to do this long term and he doesn't seem to be in any hurry to replace me. How can I step away from this work without completely leaving them in the lurch? Do I have any liability to complete the partially finished work which likely has bugs in it? I've intentionally operated as much as possible as a straight coder without wanting to take on any additional responsibility since it was side work.