@Random Musings -- do a little reading on Backdoor Roths. The taxation is based on what percentage of your Traditional IRA assets were deductible. By having a large amount in a Traditional IRA, the OP would pay taxes on almost the entire conversion amount taxed again in addition to having already paid taxes on the Traditional IRA contribution. They would also have to track basis in their Traditional IRAs from now on, which will be a tax hassle.
These issues on top of a rotten 401k? I'd say not worth it, personally. If you have any SE income, look into opening an Individual 401k and rolling the IRA into that. (Vanguard doesn't currently allow rollovers to their Individual 401k accounts, but Fidelity reputedly does.) Otherwise, use the 401k up to the max, but don't bother with the rollover.
I'm not a financial advisor, I just play one on the Internet.