It is an interesting fix, an you may be onto something, I am also having infinite middleware processes being spawn over time pushing the file limits, at first I didn't notice the excessive proccesses and just increased the FD Limit, this caused the middleware processes to go rampant and to consume all ram available eventually causing applications to fail.
I tried to apply the fix that you propose but my system failed to start at all and running "systemctl status websocat.service" returns Unit "websocat.service could not be found."
Sadly I am not as knowledgeable on truenas or this level of linux systems, so I am unaware how common this is.
Hopefully a fix can be found soon, for now I will keep the 1024 FD Limit as it keeps my system stable