I can only give you my thoughts on your situation.
As you entered UK in September 2016 and got your pre-settled status in October 2019 means that you qualify for settled status on September 2021 (5 years).
Since you ‘ve left in April 2020 you are still in the 5 year qualifying period for potentially being qualified for settled status. You say that “it has been 6 months since you left” so I would assume you are still not back in the UK. This means you ‘ve exceeded the maximum absence period of 6 months.
Now lets say that you come back to the UK in November 2020. You will be able to qualify for settled status on November 2025 as your qualifying period has been reset and you no longer qualify for settled status on Sptember 2021. Your pre-settled status is still valid but expires in October 2024 (prior November 2025) since it was obtained in October 2019. Therefore you would have to re-apply for pre-settled status as long as you ‘re back I would say and before 30 June 2021 given that you started living in the UK before 31st of December 2020.
This is how I can understand it works but in any case I would advise you to contact either the British Embassy in Slovenia or the authorities in the UK.