Is the code you ran on your OpenBSD available (e.g. on GitHub) for others to test?
Curious what async issue you faced, did you report it? Or ask for help addressing?
I just switched to single thread and didn't try to fix the issue. Single thread is fast enough to me, it has throughput ~ 730 Mbits/s in a OpenBSD 7.8 vm on a 7th gen i7 linux kvm host.
https://github.com/infinet/rs-wgobfs/commit/c5e62796