I'm not sure if this answers your question, but if you have access to a Roth 401k, it can be one of the best opportunities as compared to a Roth IRA. You can contribute more than the "normal" $5500. There are also no income limits, as there are with Roth IRA contributions. Of course, there are no income limits on conversions from IRA to Roths.
If you are still working at the company, they usually won't let you roll your 401k over to an IRA product, whether it is a Roth or a Traditional. The exceptions being that some companies allow inservice distributions. You would have to check your SPD (Summary Plan Document) or with your administrator. However, if you are planning on leaving the company, you've outlined it perfectly.
If you are eligible to contribute to a Roth IRA, I would go ahead and open it now. That starts the 5 year clock ticking. You'll likely have to put something in it. Then, you can contribute to a Roth 401k, max it out for 5 years, quit work, then roll over your Roth 401k into the Roth IRA. Then you would be eligible to take the contributions (not the gains). I would keep careful records (your statements will suffice) to help your eventual tax planning, as your Roth IRA custodian will likely not be able to help you on your contributions vs. gains at the 401k.