Also, I'm getting

e: There are some plugins incompatible with K2 compiler:
  org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationComponentRegistrar

Am I reading https://youtrack.jetbrains.com/issue/KT-49983/Implement-prototype-of-kotlinx.serialization-for-K2-compiler correctly that this should be addressed with Kotlin 1.8.20?

Yes,

kotlinx.serialization

is supported only starting from 1.8.20