I remembered something...
When I was search about set memory limits I found this: Resource control with systemd • /dev/schnouki
At that time I test but don´t worked, because, I think, the cgroup_enable=memory was not set. Maybe this is a solution for us!
This consist on set a memory limit for a group of services. I think this way we can set something like MemoryMax=400Mb for Sonarr, Radarr and Jackett.
I think if we set this, when Jackett have to update it will crash another services (sonarr, radarr) and after this back to normal!
I don't know if work this way, but maybe it work
I have set MemoryMax=300M for Jackett to enable auto-update
Perhaps is better test this way now and see the results before try this.