You described the Megabackdoor Roth. You contribute after-tax (not Roth) to your 401K, then convert it into your Roth IRA.
Regular backdoor is similar procedure, but it is done entirely within your IRAs to get around the income limits on Roth IRA contributions, which as VoteCthulu pointed out, you are under, so no need to worry about this. If you ever find yourself over the MAGI limit for Roth IRA contributions, please read up on the backdoor Roth before doing it - there are some gotchas there, particularly given your employer-provided retirement is a SIMPLE.
Also - SIMPLE IRA is always deductible. Your W-2 wages already have this excluded for income tax purposes. So when you see SIMPLE, think 401k - works about the same way. Unless the fees are truly ridiculous, this would be a good thing to max up to the $12,500 you're allowed.