You can upgrade directly to Photoshop and Premiere Elements 2020 from Elements 2018 without having to purchase the 2019 version.
The files you created using the 2018 would work with the 2020 version and you can upgrade your Elements Organizer 2018 catalog to the latest version as well. It would automatically take you through the catalog upgrade when you open the catalog for the first time with the latest version.
You can download the applications using the link below and try the applications out for a few days if you wish to ensure that the applications work fine for you and then make the upgrade.
The recent Elements programs are called PSE2018, PSE2019 and PSE2020. If you look at the system description, their version numbers are 16, 17 and 18. So you have PSE2018 which is version 16. Marketing decision...