openmediavault-writecache 8.1 is in the repo. It doesn't fix the flush on shutdown issue when using tmpfs (called system memory in the plugin) but it is working correctly with shared folder storage. It is working correctly on my six systems.
I recommend enabling "flush on boot" and "hourly flush" when using disk storage. When creating the shared folder, I recommend these settings:
Don't use a non-native linux filesystem or mergerfs pool. And use a disk that doesn't wear out quickly from writes.