Two suggestions for you. One is to not change the theme but change the plasma style. you can get one from any other theme that looks the same as you have but is kde 6.
The other thing you can try is while still in beta for 6. kwin-effect-rounded-corners-git now since it is a git an sometimes updates might break things i recommend once you get it going to add it to your ignore package list. As the hdr commit kinda messed things up for me tell I downgraded and locked it.
*I linked the gihub make sure to give it a quick read as there is one setting worth setting up.
**One other thing you can get is force blur for kde 6 an you can define the corner radius.