Hey Jack! No worries, these things often take some time to grasp. It gets easier!
The January deadline is for the full tax year before, not the one you're currently in.
So for example, you'd pay for 20/21 (April 2020-April 2021) in January 2022.
If HMRC confirm you don't need to do a 20/21 Self Assessment, you won't need to pay this January coming (2022), but for your 21/22, you'll need to pay it in January 2023.
Obviously if you wanted to be SUPER-efficient, you could submit it in April 2022, pay it by May, and then put your feet up.
Re: the £1000 'allowance' - this continues to apply, even if you have PAYE income. The £1000 relates specifically to the first £1000 of Sole Trade income - other income doesn't impact this.
Hope this makes sense!