Correct, nothing wrong with contributing for 2018 now and converting to Roth immediately thereafter. There is no limit to the number of Roth conversions that you can do in a given year. Keep in mind that Roth conversion performed in 2018 and later cannot be recharacterized back to a traditional IRA.
Correct regarding the basis. On your 2018 Form 8606, line 1 will have $5,500 and line 2 will have $5,500, so line 3 will have $11,000. As long as you don't do any rollovers from employer 401(k), 403(b) or 457(b) plans that would cause you to have a nonzero balance in traditional IRAs on December 31, 2018, your entire $11,000 of basis will be applied to your Roth conversions.
