How can I recreate my existing desktop in Bionic

[Grobbekee]

I liked how KDE 4 looked in Trusty Tahr:

[claydoh]

There are desktop Look-And-Feel packs that get you pretty close - click the Get New Looks on that section in system settings and search for Oxygen Future for one example that is mostly complete.

You may need to look for Oxygen / Air / KDE 4 styles for Desktop Theme, Color Schemes, Icons, and Window Decorations. The Look and Feel packs like Oxygen Future are supposed to contain everything all in one place, but you can try out different things for each element if the main pack is missing something or you want to tweak it. All the themeing elements are found and configured the same way as in KDE4 in System Settings

[ringel05]

I liked how KDE 4 looked in Trusty Tahr:

The theme you were using probably has not been updated to work on Plasma 5, you can create your own or wait and see if the one you prefer is updated.