There is an initial port of KioFuse to KF5 at https://github.com/bosim/kiofuse
I tried to make it work, but failed to understand it, mostly because it uses separate threads, event loops, and helper classes which pass stuff around using invokeMethod. Also, much code is commented out, and I had a hard time understanding if it is no longer needed in the KF5 port, or if it still must be enabled again.
Checking the FUSE project, I found out that libfuse3 offers a faster low-level API that can be integrated into custom event loops. An initial prototype (that does not work yet) can be found at https://github.com/cfeck/fuse-kio
Let's investigate if enhancing this prototype is easier than working with the existing code base.