If you've built up a decent balance in your 401k, have you considered taking a loan from it as a down payment? I know mine allows a loan of 50% of the vested balance that gets repaid directly from your paycheck.
You briefly mentioned a roth as well. Roth IRA contributions are available for withdrawal at any time. If you touch the earnings, though, you'll be subject to tax and 10% penalty.
Give it some time with the girlfriend. Mine wasn't a big fan of the house hacking idea at first either. 2 things changed her mind (in my opinion).
1- She saw me constantly reading bigger pockets and would ask what I was reading. I would give her examples of others that had success with it. The idea of living for free or close to it became appealing.
2- She saw people she knew in real life that are currently house hacking or did in the past. She babysits for a family while finishing law school. It didn't dawn on her until 6 months after I told her about house hacking that they were doing exactly that.
I think there can be a misconception that rental properties are only in bad areas. You CAN have the best of both worlds- a tenant that helps cover the mortgage and a desirable neighborhood.