OOM is a feature, not a bug. It’s good that your system starts killing processes and requires reboots, because it alerts you to a problem.
Have you ever considered upgrading from 32GB of RAM to 128GB? 
I’m still on TrueNAS Core, where I don’t have the benefit of OOM, poor memory management, and spontaneous reboots. Every day I have to suffer with the reminder that Core runs on the inferior and boring FreeBSD. 
How lucky you are.