Most of the benefits of converting a traditional IRA to a Roth through the "backdoor" method like you mentioned are lost if you have existing pre-tax assets. Look up "pro rata rule" to get more details on tax treatment when you convert.
How much higher do you think your tax bracket will be in retirement? A lot higher, or just a little (within 5% or so)?
If a lot, it may be worth converting ALL your assets to Roth and paying the taxes upfront.
If a little, I would contribute fully $17.5k to your 401k, $5.5k to a Roth, and then the remainder of your investments in a taxable account.
The benefit of having multiple account types is that it gives you options. For example, you could put your high-income (things like REITs and bonds) into your Roth, broadly invest in ETFs and whatever is available in your 401k, and then invest as you wish in your taxable account.