Well, the pro-rata calculation for a traditional IRA distribution in 2020, including a Roth conversion, depends on your December 31, 2020 balance in traditional IRAs. As long as you don't move any pre-tax money back to your traditional IRAs before the end of 2020, the taxable amount of your Roth conversions will only be the amount of any investment gain during the short time the money is in your traditional IRA.
Assuming that you are eligible to contribute to a traditional IRA for 2020 as well as for 2019, there is no reason not to make both your 2019 and 2020 traditional IRA contributions on February 1, 2020 and convert the entire amount to Roth at once. Just make sure that you still have a $0 traditional IRA balance on December 31, 2020. Your nondeductible contribution for 2019 will appear on 2019 Form 8606 Part I line 1 and will be included in the sum on line 14. Line 14 of your 2019 Form 8606 will transfer to line 2 of your 2020 Form 8606 to be added to the amount of your contribution for 2020 on line 1. The result will be applied to your Roth conversion to determine the taxable amount of the conversion.
Since you made no distributions from your traditional IRAs in 2019 that you did not fully roll over, your December 31, 2019 balance is irrelevant.
